Thanks everyone for the comments. I will be purchasing a fuel pressure gauge to monitor the values. It's a relatively inexpensive and useful item. Will keep everyone updated on the results.
 
On another note:
 
Doug kindly provided me with his MAP sensor to test in my car. Stumbled 2 out of 3 first starts now. The new circuit opening relay arrived this afternoon so I will drop that in the next few days to test.

Update to the thread. The MAP sensor was ruled out, the car still stumbled with the borrowed sensor - no improvement. Stumbled 4 out of 5 starts.
 
Circuit Opening Relay: I have had 10 good first starts in a row since installing the new Circuit Opening Relay (10th start being this morning). It's the best result I have seen with any testing. While not 100% indicative that the issue is fixed, it's a good indication that it could be the solution.

Thanks for the updates Lumix.  Easy fix if it's just the COR...

Sounds like you may have found the culprit :) Hope it keeps starting normal for you!

As we discussed give it a week and then swap the old COR. back in and see if the stumbles return again.
How did you go with fitting the fuel pressure gauge ?

xez
Thanks for the updates Lumix.  Easy fix if it's just the COR...

Mrskylighter
Sounds like you may have found the culprit :) Hope it keeps starting normal for you!

 
I hope so - I will keep everyone updated. Started great this morning.
 

Falcon
As we discussed give it a week and then swap the old COR. back in and see if the stumbles return again.
How did you go with fitting the fuel pressure gauge ?

I haven't installed the fuel pressure gauge yet. I serviced the car Saturday afternoon before it got dark. Plan to get the new fuel filter and gauge installed this coming weekend.

Update
 
Car stumbled after 16 days straight of perfect starts... The only thing that had changed was the newly installed fuel pressure gauge and fuel filter. After installing the new fuel filter and gauge, fired up perfect, drove the car around later that night, parked it up and started it 6hrs later on Sunday Morning to meet the MR2 QLD club and it stumbled. Fuel pressure before starting was 0 psi as it was stumbling the pressure was steady at 44 psi.
 
This morning (Monday) fuel pressure was 0 psi, the car was started while I watched the gauge. I had instant fuel pressure to 44 psi and the car started perfect.
 
Reading through the service manual at the moment to check the specs

Leaky injector maybe as fuel pressure drops to zero overnight? Too much fuel in a cylinder causing a temporary misfire at cold start?

Maybe see how long it takes for the pressure to drop?

Bummer, I thought you fixed it.

Mrskylighter
Leaky injector maybe as fuel pressure drops to zero overnight? Too much fuel in a cylinder causing a temporary misfire at cold start?

Maybe see how long it takes for the pressure to drop?

Bummer, I thought you fixed it.

I had the injectors tested, cleaned and serviced in April last year, no leaks. I will monitor the pressure drop.
 
Same, I was starting to get excited.

Good luck with it man.
 
I'm inspired by your filter-adaptor-gauge purchase so have ordered those parts for own car as I feel it is running a bit too rich overall across the entire rev range. 
 
I ordered two Blox gauge adaptors from the US so I'll have a spare in case someone else needs one.