Glen     I would be highly suspicious of your new oversize inlet valves!!    I don't know what material or make they are but I suspect they could be stretching.  Stem stretch.  I have seen it and usually occurs at first really high heat episode just as you have described.  Tipping is just grinding the stem tips all to the same length to get into the range of the valve clearance adjusters. Shims in this case.   Glen your cylinder head guy should know the length of your valves originally so if you measure them again now and they are the same you can disregard valve stem stretch!!     Wish you well with this.  I can see you have put in a lot of effort and I like your driving style too. Keep us posted how this goes.   PM me for my phone no if you want to have a yarn about this.

Yes that is a possibility. They are Ferrea Competition Plus Oversize Valves 25º Super Flo. 1 mm oversize. Super Alloy. I bought them from ATS Racing.
If they have stretched, do you think that they will continue to stretch?
I will be re-shimming them and driving the car again anyway though of course I would like to understand what has happened. Implementing workarounds without understanding the root cause often ends in pain.

Here's another theory - http://www.vw-resource.com/heads_valves.html
The clearances were set with the head off the block. Maybe the torqueing of the head studs was enough to affect the valve clearances? As I said, the idle was always bad with this engine so I suspect that the inlet valves were always a little bit open.
I read another forum thread where a guy bought a Honda dirt bike and the idle was bad from day 1. He eventually figured out that his valve clearances were wrong.

I was thinking about your problem last night and it definitely can't be timing or cam sprocket related because your saying that the clearance was zero all the time, not just when measuring from TDC. So your valves were never closing fully ever.
Yeah valve stretch is a possibility, basically your valves have either moved higher, got longer or your camshaft has dropped lower, it can only really be one of these three things. I've never heard of valve stretch first hand, would it happen to every intake valve though, they all stretch the same amount? If your valve clearences were wrong initially they have somehow managed to get worse? Usually they get larger not smaller. It's a really odd problem your describing. Is the cam carrier alright? That hasn't worn and your cam dropped down? Also what is your wipe area like on the rocker arms?
*** EDIT***
Actually 3S is shim under bucket don't worry about rocker arms....

I started reading up on valve seats receding into the head just out of curiosity as its such an unusual problem your describing and came across this thread that was interesting.
http://www.thumpertalk.com/topic/42778-anyone-have-an-intake-valve-stretch/#!F27Bb
Basically the guys valves were spinning around and wore into the valve seats. He only had a few valves go though not all.
Would be great to find out what the cause is, be sure to post up if you find it.

kameleon
Tipping valves? is that some crazy name for lapping the valves?

Yes

track_mr2
I started reading up on valve seats receding into the head just out of curiosity as its such an unusual problem your describing and came across this thread that was interesting.
http://www.thumpertalk.com/topic/42778-anyone-have-an-intake-valve-stretch/#!F27Bb
Basically the guys valves were spinning around and wore into the valve seats. He only had a few valves go though not all.
Would be great to find out what the cause is, be sure to post up if you find it.

Yes I did read that one earlier today. I'll definitely let you all know the outcome. Thanks for all your help guys!

If there is no clearance when they are cold then you are losing compression when the car gets hot that's seems a natural path as the everything expands under heat. My exhaust valves are set at the end of the clearance range due to the heat generated under high temp conditions that you find at the track. When it cools down, the valves could stay at this 'hot' length giving the perception of 'stretching'. Could be growing, like my gut..
Can you check the compression today as the engine is cold. It will be interesting to see the difference from when you checked it when it was hot at the track.
Another point, if you changed the valve seats, could they have moved? Could the seats have stretched/spread with the larger valves?
At the end of the day, its great to see you are at the track every five minutes!

B24
Can you check the compression today as the engine is cold. It will be interesting to see the difference from when you checked it when it was hot at the track.

The 75psi is cold

B24
Another point, if you changed the valve seats, could they have moved? Could the seats have stretched/spread with the larger valves?

I'd have to take the head off to find out. I really don't want to do that. I'd rather re-shim it and then drive the car until it breaks.
 

This is interesting - http://www.mr2project.com/shimless-buckets/