While i'm driving down the road at steady throttle, the motor will surge up about 300~400 rpm's. I hadn't noticed it until today after I had my flowmaster installed.

I heard it revving up a little as I was cruising at a steady speed down the highway and started watching it closely. It does it consistently, a few seconds between each surge.

Any ideas on what could be causing this?

 

I'm having the same issue. Try the TPS.