Another tach issue thread

I've had my Autometer autogage 5" tach in the dash for the past couple of months and have had this issue since i installed it. the tach has an intermittant problem under 2K rpm. it usually jumps around between idle and 3K anytime i'm under 2K. i've dealt with it though since i am rarely under 2K rpm. i thought that it was normal for an aftermarket tach, but then i installed a 3 3/4 sunpro in the wife's 510 and it works flawlessly.

the reason i installed this tach in the first place was because the stock tach would only rev to around 3K rpm... very odd. i attributed it to the fact that it has a 280ZX electronic dizzy in it. anyway, what could be causing this? is it possibly a defective tach, or am i missing something?

i should also add that when i have my lights on it usually acts ok, but when i turn my turn signals on the needle bounces along with the blinking.