Jumpy aftermarket tach

Ok, this tach isn't on an AMC vehicle, but it's an older vehicle that may be used to visit AMC shows, and is at Hershey every year. 5-10 years ago I put a small aftermarket tach on my '73 Dodge RV, it's a 360 w/electronic ignition. Pretty simple wiring, 2 wires to power(tach and light), a ground, and a lead to the coil. A few weeks back the camper developed a no charge condition that resulted in me doing a lot of surgery to repair high resistance connections and cleaning up grounds. After getting the charging back, it was overcharging, and I traced that to a bad battery that acted fine but was demanding too much current(bulged out on the sides). replaced the battery and overcharge condition went away. But now the tach tends to bounce or jump every few seconds or so. I've also replaced the voltage regulator. So why the jump? Did I cook the delicate electronic innards during the overcharge condition, or is there something evil happening still in the charging system? A voltmeter on the battery shows the appropriate voltage w/no spikes while the engine is running, 13.4 to 14.5, down from 15-16 during the overcharge condition. Only ran for testing during the overcharge condition, maybe 15-20 minutes total over 2 days in 2-3 minute intervals. Ideas? Just don't want to cook anything else. Edit- I replaced the ballast resistor as well, the old one was 37 years old and cracked, for $5 I figured why not. All connections are good.

Edited by Pat D - Jul/24/2010 at 4:18pm