it sounds like heatsoak to me! Check your intercoolers, the problem may be as simple as that!

 

yeah these cars are real suceptible to heat! I am not looking forward to the carolina hot days coming up!!! I'lll be losing a few races
Gotta get me some RS4 intercoolers
When my intercooler hsoes was popping off, the car was a mess like yours, power on and off...
check them out

 

Well we were in the lower 50s here in south FL because of a cold front... same thing doesnt matter the temperature...

 

I second whomever said to take a look at your diverter valves. Though it would be a little odd for them to cycle like that, it's possible if one was failing or the control line split or was leaking. While you have the engine covers off, take a good look at the TBB as well; they're known to fail and crack, which would cause a compressed air leak.