Tach dropping to 0 while still running almost sounds like a crank position sensor starting to go. If so, then soon your car may start just randomly dying on you until it just won't fire up anymore once the sensor is shot.

Barring that could be some corrosion on the battery cables. Remove them both, clean the batt. and the cable ends, put a little dielectric grease on them and reattach making sure they are tight. I've seen loose/dirty cables cause this too.