when i turn my ignition to power, i can adjust my tach to when i want the shift light to light up. then when i start my car, it will go back down and run with the cars rpm guage. however, these past few days when i am sitting at a red light or in traffic, my needle on my aftyermarket tach will jump up and down to where i had it set. it goes up and down relly fast, then when i give it gas again it goes back to normal with the cars rpm guage. whats going on. any help please. thanks.

 

what kind of tach is it? autometer? if so return it to them, sounds like it is going defective if you ask me.. never had the problem w/ my tach, and hope i dont ever..

 

Sounds like a loose connection to me, I've seen this problem before in the aftermarket tachs I've owned in the past (both AutoMeter and cheap ones).

 

check your ground