starting issue

We have a 70 Dart 360/727 with Hooker Competition headers. It hasn't been completed that long but I have noticed that a few times after it has gotten hot, upon restart the starter spins very slow. In those instances, after a few minutes cooling down, the starter spins freely and it starts right back up. I feel that this occurs because of the headers being so close to the mini-starter. Note: I took it on a 90 mile round trip last week and did not have this issue. I assume it was because it was primarily highway driving and limited stop and go driving. I know that this is not exactly your issue but just thought it would be helpful. If this continues, I will be picking up some heat wrap for the starter.
As for thoughts on your issue, I agree with Petty. If you can jump the solenoid and the starter engages like it should then the problem has to be in the ignition IMO. Good luck.