Mystery - higher RPM break up only when hot. 5.0 EFI in '70

Hey - I was hoping someone might help solve this mystery.

My new longblock started acting up after about 8,000 miles of running perfectly.

When the engine is cold, it idles perfectly and if you floor it, it just roars smoothly through the gears with plenty of power. It's very smooth at any RPM.

Once it gets close to operating temp, every few seconds it skips when idling. If you open the throttle too quickly, it loses power and continuously stutters almost like it's going to backfire.

So if I want to go, say 70, I can work up to it no problem, but giving it too much throttle at once makes it really unhappy.

My setup: new stock 5.0 long block, stock used EFI setup, new vortech external fuel pump, c4 transmisison, full length headers with O2 sensors, new cap & rotor, happens to be in a 1970 Mustang

I have some ideas, but the puzzling part is it seems temperature related rather than RPM related (ignition) or load related (fuel).

Thanks for any advice.

Hey - thanks!

I had heard the first sign the TFI module is dying is the car won't start when it's hot.

So if the spark is falling off as it heats up, I can see getting what sounds like detonation or maybe unburned fuel popping off in the exhaust.

I'm swapping in the dist. from my other 5.0 just to test it out.