It could also be a bad crimp or a broken wire, not just corrosion. Check all your crimps, connections and wires.
My ammeter would show either a red-X or +99amps once in a while. This started happening after 300hrs of flawless operation. I found bad crimps where the wires connected to the shunt causing the problem. Once I re-did the crimp the problem went away.
I also had a similar, intermittent issue with my #2 EGT signal. But this one worked fine whenever I tried to trouble shoot the problem. The problem with that one was a bad connection inside the EGT probe connector. Additionally, when the cowl was on, the #2 EGT wire was being bent enough that it would make a bad contact. When the cowl was off, the plug made a good enough connection and it worked. I re-did the connection and moved the wire so the cowl would not interfere with it. Problem solved.
Good luck on the troubleshooting. These kinds of problems are a b**ch to find.