1990 Ford Probe loss of power.

I have a 1990 ford probe i turned it in to a race car. but i didn't change the motor. i can go about 10 laps then the car get to 210 degrees and then the car start sputtering. It want hardley go. i cant get it on the trailor until it cools down. but it never overheats. I,ve already changed the coil.O2 censor. The fan is wired to run all of the time. It is lossing fire somewhere and i can't find it. after the car cools down it runs again. i can run in second gear all day but third gear puts a strain on it when it gets to sputtering.