Hi Jon,
From what you've described, I'd be looking at either the TPS (best test is to swap in another one from a friend) or the ECU coolant temp sensor - they are very cheap, you could probably just buy another one to try it. If the coolant sensor is giving duff information, then it's probably telling the ECU that the engine is cooler than it is, and as a result you're getting bogged down by too much fuel - how's your MPG? Does the exhaust stink of petrol?
In terms of logical diagnostic tests, I always start from the basic premise that the engine needs 3 things (in the right proportions, at the right time) to run:
fuel
air
spark
Now it sounds like you can reproduce the problem whenever you want, so presumably you can reproduce it once the engine is warm with the bonnet off by rotating the throttle spindle with your hand.
Test #1: reproduce the problem as described above, then try doing it again while spraying Easistart into the air-intake (remove air-filter). Does the problem disappear?
If the problem disappears then you have too-little fuel going in (or too much air - air leak?), if it doesn't then the problem is either caused by too much fuel, or lack of spark... if you report back the results, we can try and devise the next test...