I know it's probably electrical noise, but I've wired up this tach I picked up yesterday (new, aftermarket) strait to the battery + and - terminals, and the coil-side -, and it still jumps as soon as the engine goes about about 1,200 rpm. It seems to work well and fine below there, increases steadily to 1,200 and then just decides to start guessing at the rpm and jumps between 2,000 and 7,000. Any ideas? Bad condenser or something distributor side maybe?