e30 m3 tach is sticky when cold, and somtimes warm

I have a 88 m3 and i have been noticing my tach has been sticking a lot when cold, but after 5-10min of driving it loosens up and works like normal, but a couple of times when reving high it got stuck at 4.5 k rpm and didn't know my engine speed any more,but a second later went to normal. i think i should fix it because the tach is one of the most important gauges. does any one know that problem??? maybe worn out tach, or bad signal, the car has 140k miles on it, so it might be worn. does any one know where can get a new tach if thats the problem.

thanks Tony

[edwinelisia]

where does the tach sensor goes? maybe you want to trace back from that sensor back to your tach