I have a pair of 500efi's and the livorsi tachs jump all over around 3000 rpm on both engines. Under 3000 they work fine. I just changed to brand new tachs as I thought they were the problem. The result was the same. Anyone have this problem before?

I'd check the grounds real good, and make sure the tach signal wires aren't getting interference from anything routed next to them.

Unfortunately anything more than replacing something electrical with an identical item is like chineese to me. I can do mechanical stuff no problem but electrical I suck at!

I had the exact same issues with a 358 Sonic.I had one tach fail and couldn't get the replacement to sit still over 3000 rpm. I went back and forth with Gaffrig and probably tried 4 or 5 different tachs before I finally got them to operate correctly. They were baffled too and sent me all different wiring configurations before we found the right ones. I'd call them and tell them the issue. They were great with my problem

Do you have factory plug wires? I know you didn't mention anything about doing any other work then the tachs but I've witnessed the exact thing you mention when a friend had a 500 efi and he ran the 8.8 wires instead of the factory merc wires. Maybe something with the resistance? It was the only thing he changed on the motors, put a brand new set of factory wires back on and tach was "normal" again. Not an expert opinion just my .0000002

Plug wires can do it.

It has moroso wires. Factory merc wires might help?

I'd hate to tell you to go spend 150+ on two sets of wires, but maybe buy a set try them on one motor? If it works then buy a second set. If they don't work neatly put them back in the box and plead the 5th?

I had the same problem on one of my 500 efi's it happened to be one of the wires on top of the coil that was defective. Crank the engine and jiggle wires on the coil and the distributor to see if there is a change in engine rpm or sound. It's definitely electrical problem though.

Poor connections.. Grounds, powers or signal. Any could be it. Your boat sits in the winter? Had same issue here in MI. Moisture.... Corrosion.

Just my thoughts.