Autometer tach having intermittent reading

[87Predictor]

I'm running an 87 Johnson 120. Put new carbon fiber pro comp marine gauges in. All gauges work fine but the tach begins to freak out around 5500. If I hit the trim button it may drop to zero, if I back off the throttle is may jump back up. It's just real intermittent when its up over 5 thousand.
I never ran the boat with the old gauges in so I can't say its the new tach that's at fault.
The tach only goes to 6000 rpm. But I think it should give a consistent reading all the way threw the range. All connections to the tach are soldered and shrink tubed.

Can someone advise me? I've tried searching the Google. Never owned an outboard boat before. But the term rectifier/regulator came up.

Volt gauge reads a good 13 volts while going. Running 2 batteries and a perko switch.

Help! I'm turning this motor to pretty much the end of the tachs scale and i would like it to read rpm without hiccup up there.

Thanks,
Melvin

[87Predictor]

Anyone? Point me in the direction.