Yeah, the new ABV is for the handling bigger pulley without doing a "dodgy" bypass job. :p I've never felt it kicking in as i've never gone past 8psi on the stock unit, so i can't tell you if it's the ABV activating too low, but something must be responding to either the boost, or knock (caused by the jump in intake charge temps), and reigning everything in.
 
It's hard to diagnose without having a poke around. Does it happen after a couple of seconds of WOT when the engine is warm, or instantly? If it's after a few seconds i would suspect the knock sensor is seeing something. If it takes a much longer time when the engine is cold, i would say it's definitely the intake temps or a faulty knock sensor (have to check the temps). Is the SC good on oil level?
 
If it's the instant the boost peaks, then it's boost cut (wind the abv up a bit).
 
When i have some free time i'd be happy to come and check it out. I'd like to see what 11psi would be like in my car. ;D

I have spoken with Terry from Road & Track who believes this issue could be fuel related. So I set about last night to see if there are any obvious faults in that area. Here are my findings:
I attached a lead and LED to the FP connection in the Diag box. When the FP is on it lights up. This was done to check the switch in the AFM was working correctly. The led illuminated as soon as the engine started to crank and did not falter at all – so the FP and AFM switch are working.
I replaced all the vacuum hoses and made sure there were no leaks. Still no difference.
I also tried to test the Fuel Pressure regulator and it appeared to be ok (although if anyone knows of an easy way to test the FPR then let me know..)
I’m pretty sure it’s not the knock sensor either .
So I appear to be no closer to finding the cause and solution….
 
And to answer CUBITS questions, it happens only when warm(ish)  - I can drive up our street into 3rd gear - all good, the instance I do a U turn and head home it starts ...
SC Oil is good.

Have you tested the fuel pressure?
Low fuel pressure may cause some of the symptoms that you have mentioned.
eg. faulty fuel pump or partially blocked fuel filter.
 
I'm not sure about the temperature (warmish part), as driving down the street and up is a very short time, if it was a lengthier drive then I would suggest to have a look at the coolant temperature sensor, you can measure the resistance vs temp and confirm if it working. But I would say check the fuel pressure first.
 
Regard the ABV, if it's a stock ABV with a bolt on it, it maybe a modification to increase the spring rate to hold the pressure during boost. This is just my thought as I haven't hacked one of the ABV open before.
 
I guess some photos of your setup and ABV would be good start for others to help!
 
= )
 
 
 
 
 

Your ABV looks like this??
 

 
That's a cusco one. The other telltail signs are the tops are different, as a stock one has a press bent top.
 
could be fuel / timing related...

THATS IT !!!!!! What can you tell me about it ? - How do you adjust it etc ???
 
I'll throw a new fuel pump and filter in it this weekend and check the fuel pressure too..
 

Yeah, it is a cusco adjustable bypass valve. What happens is instead of bypassing the vsv's that normally control the factory vsv, this allows you to adjust the point that the bypass valve opens. the nut at the bottom locks the bolt in place, and the bolt can be wound in to increase boost levels, or wound out to lower them (this is provided you have a pully setup to make the boost in the first place)
 
You will probably find if you unwind the bolt two turns, your boost level should drop. If it doesn't, there is a good chance is it over-preloaded, which may cause some running issues (tho unlikely the ones you are experiencing.) This bolt should be adjusted to work at the boost that you want. Maybe try taking down the preload, dropping the boost to say 8 or 9 psi, and see if you still have the cutting out problem.

Yes, that is definitely a Cusco adjustable abv, so don't toss it out, lol. Those are worth money! They are designed to allow an adjustable boost level, as mentioned, but also allow the sc to spin up a bit more normally, as throttle is applied. Basically, the system will act more like a stock system than if just the abv mod is performed. It is not necessarily a huge difference, but is nice to know everything is working as designed. What I like about them is that you can essentially dial out over-heated boosting, by checking your max boost when the engine is cool, and then setting the abv for that boost level. Anything above it will mostly be wasted over heated boost that eventually does more damage than good.
 
As for you real issue though, it currently sounds like you have what is known over here as the low-end lean out, although your initial problem sounded more like the jdm top end issue that is also known.
 
The low end lean-out is usually caused on non-jdm 4agze engines/ecu's that have different ecu maps than the jdm versions, and are being over-boosted. The issue is more apparent under wide open throttle at low rpm, and causes a massive lean out issue and stumbling, before regaining its proper fueling at around 4k rpm. You can test that easly enough by lowering the boost level with the Cusco down to around 8-9 psi as mentioned previously. If that solves the issue than you most likely have a non-jdm ecu that is hampered by over boosting.  The solution would be to either get an aftermarket ecu/stand alone, or get, what is known in the US as a Grunt Box, a device made to use the CSI to provide extra fuel at the point of lean out.
 
The initial issue you mentioned, however, the 5,500 rpm one, sounds like the jdm upper rpm problem that is caused by knock issues, and I believe is found with slightly newer jdm ecu's than what came in the MKI MR2 originally. When over boosting, the knock signal overwhelms the ecu and cause a dramatic drop in timing temporarily, until the rpm's drop and the ecu can correct itself. There has never been an easy solution to that issue that I am aware of, other than going with a stand alone ecu.

Thanks for the info - I have decieded to "go for the doctor" and so a complete race spec rebuild - Including a Haltech Sport 1000 ECU! .... I think my issue will go away without me ever really knowing the true cause!. ... :)

just wondering where i can get a grunt box?

http://www.gruntbox.com/
 
jury is out on whether it only makes a difference on usdm ecu's or jdm as well but that site should get you started.