I purchased a 5K tachometer (and console gages) from Classic Industries for my '67 RS 327. I replaced the under dash and engine harness and clipped the appropriate connector to the distributer. The tach worked fine for a few weeks. Now when I rev the engine it is smooth until about 3500 rpm where the needle seems to waiver and can even decrease a bit. I don't have an factory original distributer. It is not an MSD type but I'm not sure what kind it is other than the cap is red. The tach wire connection at the distributer it is tight and the engine raps out very easily without loss of power. Any ideas what would cause this?