tach problems
 

I have an 01'370ss with 502mpi's. It came factory with thru hull exhaust/ captains call. I just recently changed out the factory exhaust to Stainless marine. I also fattened up the computers thru Arizona speed and installed 1.8 roller rockers. Other than that the motors are stock. Now the problem that I'm having is that when my tachs get above 3000rpm they start bouncing all over the place. The motors are running fine as well as the other guages are working properly. My mechanic is showing it as some sort of RF problem (radio frequency). I've checked all the grounds,changed the plug wires. VDO suggested noise diodes on the lead tach wires. None of these have worked. Has anybody ever come across this problem ???