For what it is worth I agree with all the above comments, but I have a question or two that might help.

When the car is on the track are you using an alternator? In my day we did not. We put a couple deep cycle batteries in the back to help with traction and would run everything frm that. I am getting at two things. Maybe the battery is just low enough not to satuarate the FET or if you are using an alternator by the time the second stage kicks in it is giving "dirty" power.

If it works on the bench or sittng in the garage, like Tim was getting at, also look for heat or mechanical stresses.