Tach Woes
My first approach on a nasty problem like this is to isolate the problem. You have lots of variables.
I'd first work on the big picture and find out if it is the bike or the tach. Beg, borrow or steal a Tach from a different manufacturer. Disconnect your tach and connect the test tach to the same wires. If the problem disappears, you know the Tach is the problem. If not, we know we need to start trouble shooting the bike wiring.
My first approach on a nasty problem like this is to isolate the problem. You have lots of variables.
I'd first work on the big picture and find out if it is the bike or the tach. Beg, borrow or steal a Tach from a different manufacturer. Disconnect your tach and connect the test tach to the same wires. If the problem disappears, you know the Tach is the problem. If not, we know we need to start trouble shooting the bike wiring.