On my Pantera, my fuel sender and main ground block would get some light corrosion at the connections and would start acting eratic. It would peg and then go back and forth to normal. The ground for your tach could be intermixed with your ground to the fuel guages or it could be a short at you main ground block behind the dash. I never had any fuse failures or other electrical problems, just guages acting goofy. I don't think its shorting out with a positive lead, just not making a good connection at some point.

Check ALL of the connections on the back of ALL of the gauges. I had one that wiggled out of the crimp connector, and 1/2 of the dash went wacky as the boat would go over bumps.

It' not his gauges or the spark plug wires. The tachs were replaced with the good Livorsi ones. It did the same thing with the factory tachs. The engines had low hours on them and I switched over to 8.5 MM MSD wires and the problem still persisted. I talked to the guy at livorsi about this problem, he sent me some small circuit boards to install on the back of the tachs it made it worse. Jim can chime in but it only happens about 4,300 RPM'S and higher unless that has changed, and one tach is worse than the other. It has ground problems somewhere.
(I used to own this boat)

Its doing it from 2500 and up now, and yes one tach is worse than the other. Going to check grounds tonight and maybe even run new ground wire for the tachs.

Wouldn't a ground issue effect all the gauges?

I had some issues similar to this that turned out to be a dead diode on an alternator....

c spray how do you test or check for bad diode in alternator?

finally went out and my gauge problem is gone seems i had a couple of bad gauges if anyone does send them in to have them checked see if they can make all the old ones match any new ones they send you, I can tell which gauges are the new ones from the different color lights

Another good place to investigate is the main bullet connector on the harness. Sometimes you just need to pull it out and push it back in and it will improve. Not sure if this is a problem on the newer merc products but it was at one time.

THere is a problem with the Merc. smartcraft comp. THe tach just shuts off or randomly fluctuates. restarting the egnine will fix it but I wonder what Merc has to say about it. They know.. I talked to them.

Well I finally solved my issue it was the plug wires, both tachs are solid as a rock now. Checked the old wires per msd two of them ohmed out bad but never had a miss on either engine.