Tach problems(erratic readings) help

[bigeasy1]

My Tach on my Tsunami,Mercruiser I/O 5.7 350 V8 is acting up.The needle swings wildly from the mid 3 to 5 thousand Rpms. when under power.This tach(a Teleflex) was installed new last season, along with all new gauges and electronics.It has less than 50 hours on it.
When it's idling,it seems to function fine,but when the engine is above 3,000 rpm it starts with the erratic readings with the needle jumping back and forth between 3 and 5k.
It worked fine for the first few months,but started acting up at the end of last years fishing(September)
I haven't had time to dig completely into it,but I did check the wiring at the gauge and it is solid(it's all tinned marine wire in perfect cond.).All the other gauges work fine,and are problem free.
I took a quick check on the engine end and all wires and connections seem to be fine.

Anyone have any ideas of what would cause this(ground looks good)or something that I should check first,before I tear it out.The gauges are all jumped from one to another as were the originals,and It's all run through the Mercruiser harness.It's a thunderbolt IV ignition around 1991 vintage.The engine is in perfect running condition.

Any thoughts or comments are appreciated.Thanks all.