NEED HELP...Tranny/Electrical Trouble

My buddy has a 2001 GTP 3800 Series. Just had SLP Headers installed. A few days later we started having trouble. While we were starting to drive again from a stop, the car would not let us drive faster than 25 mph and would not shift. Anything higher than rpm's sounded like it was just revving the engine. We did rule out the fact that it was mechanical b/c we pulled over and turned off the car, then restarted it and it was fine for about 10-15 minutes. If it was mechanical, it wouldn't have reset itself. He will be taking it in on Tuesday to the dealership to get it checked out, but I wanted to know if anyone had any ideas on what could be wrong. Thanks for your help in advance.

Check and make sure you didn't get any of the transmission control wires too close to the headers, you could be developing a short and make sure all the vacuum lines are connected especially to the trans modulator valve. Also check to make sure all connections were put back properly and are not too close to the headers. They may be overheating and will cool off enough to allow the tranny to work properly.

Looked at the wires and connections, no wires are touching the headers and all lines are connected. We disconnected all the sensors and reconnected them, still acting up. I understand about things overheating, but will the 5-10 seconds that the car is off really gonna cool it down enough to let it run properly for about 10 more minutes? We also looked at the transmission fluid, looks clean. Any more ideas on where to look?

Its a vaccum leak!

Recheck all vaccum hoses...trust me on this....this is not a tranny problem

I bet its the vaccum hose in the center near the back...AKA "Manifold Pressure"