I have a 2004 G3 HP180 running a VMax 150. Just this spring, the tach started acting jumpy and erratic. Tapping on the glass would get it to settle a bit until the next time. Had no idea what was going on until I trouble shot over the internet. Teleflex's website describes the issue as a "film" that forms between the Selector Switch contact and the circuit on the printed board. According to Teleflex, this film tends to intermittently insulate the two causing the strange readings. The suggested remedy is to rotate the switch about 6 times, resetting the switch to its original position. This is supposed to eliminate the film build up and return the tach to normal.
Want to report that I did this last weekend and today was the first time I was able to get out. Sure enough, the tach was acting perfectly fine. No jumpiness or erratic issues at all. Thought I'd pass this along to folks in case they have similar issues.