It sure sounds like a wire/connection issue. Like its not able to supply enough juice at full acceleration. just curious, does it do it when not under load (like hold the car off the ground and try it)?

or try it with no punch control. I had an issue with one of those a while back. It would screetch and stutter with ANY punch control dialed in. 0 punch control and it was fine. It now sits on the shelf collecting dust!