I've got a rebuilt 700R4 with a Transgo shift kit. Towards the end of the track (like 15 feet before the finish line) I'm in 3rd gear at about 5800rpm and all of the sudden the trans will just let go. The tach goes up and I loose power.

This only happens at the track where I've got my foot jammed to the floor, and only after a few good runs so it is hot. It never happens on the interstate.

I also put the high revving return springs in from transgo in case that was an issue.

Does anybody have an idea why 3rd lets go like that? Could it be weakening clutches already? Or weak line pressure?

 

The hi-rev springs have nothing to do with it. Did you block the 4th accumulator like it says in the instructions? Did you drill the 4th feed hole to .155"? What size boost valves are you running?