I have the same type of problem, been that way for the last few years. Except mine does it past anything above 2500/3k rpms. The needle will go up, then bounce back- it will even go below the 0. (annoying) Eventually I will get to taking the gauges apart and checking it out but I just got a autometer tach. When I posted on this before people said to just disconnect the stock tach wire, etc. I use to have msd and they said that could have caused the stock one to go bad?? Let me know if you guys figure this out I really want to get it fixed.