I just posted this question in the coroner garage, I think that was the wrong place.

I have a 99 Camaro with an Ls6 Motor and a few modifications. The car is a manual with the 6 speed. When my foot comes off the gas, the engine speeds up and then slows down and does this speed up and slow down routine for about 15 or so seconds. My first thought was a vacuum leak but I could not find one, what else could it be ?

No codes appear.

Thanks for your time and thoughts.