I have twin 502MPI's. The RH tach gitters above 4,000 rpm. below that it is rock solid. The engine runs fine but the tach gitters around 200-300 rpm. I switched tachs left to right and the problem stayed on the right. I have checked grounds and power and all seems correct. Any ideas?

Swap distributors and see if it follows. I have seen the tach start to jitter at high rpms from a loose distributor shaft. Eventually it gets worse and the engine starts to miss too.