Finally got to digging in to the tach issue. The tach has continued to work properly, then not work properly.
Windscreen and some other bits off, and the gauge cluster was out. The brass screws were quite snug but managed a slight bit of added snugging. All connections looked fine, but I slid that 3 wire connector in and out a bit. Need to get some dielectric grease. Left it on the boat.
I took this opportunity to rewire my 3 wire volt meter. Previously I had tapped the low beam power to turn on the display AND power it. Now the low beam just turns it on, and dedicated wires to the battery now power it. Previously I was losing almost a volt to the ducati low beam wiring. Now it reads full battery power.
So..I will reassemble it tonight and wait for the rain to stop. I really do not feel like I fixed anything, but maybe I did.
I tested it in the garage, and the tach behaved. But that doesn't mean it will continue to behave.