I personally would suspect a fuelling problem, however i wouldn't count on anything. Its very worth while getting a timing light and making sure your timing is correct.

Spark plug gaps generally don't change, it was running well, then started to misbehave.

Then do as Rick says above. I also find a fuel pressure gauge installed temp under the bonnet very handy, you can see exactly what the pump is doing. Air leaks can also do this as when cold there is enough extra enrichment from the cold start injector to compensate for an air leak. When the engine warms up, the bi-metal switch controlling the cold start injector turns off, the car goes very lean and won't run.

You can check this by pressing the fuel metering flap down (very slightly and slowly or it will stall) as Rick has suggested. The further down you press it, the richer the mixture will be, if this helps matters i would suspect an air leak first (as these are generally free to fix).

Common air leaks come from the rocker cover, the oil cap, the dipstick not being fully in, broken breathers etcetc. But can also be caused by loose boost hoses.

Other common MFI problems include faulty sensors, it is not unheard of for a sensor to work when cold, and go open circuit/short when hot. The coolant temp sensor will have the largest influence over fuelling.

Finally, take it to a tuner and get it set up.

Good luck

Rob,