Jumpy Tachometer

I have a problem with my tach on my 88 xj. After about 20 mins of running, when i am idling at a light, the tach will jump, like it has a "twitch" it only moves a few millimeters from where it is supposed to sit, but can anyone tell me where the stock tach recieves its signal from??? Or what could be the cause of the problem?? Thanks!

ive had this happen in two of my cars a honda Accord and a Xj in both it got worse and worse until i had to replace the instrument cluster. It works fine after that so its something in the instrument cluster. I hope it helped out just replace it thats the easier way to go its very eady to do hope that helps

Wow, this is a doner cluster already... if it gets worse, ill do something about it, but its not that bad now...thanks

Anyone else have an idea??? It is intermittent problem, and it flutters around where it should be at idle. I have checked the grounds on the cluster, and the plugs under the coil. This just started all of a sudden one day. Some days it wont jump around at all, and other days it will do it all day long, and it only does it when it is idling, so i dont think the gauge is at fault since it doesnt do it all ther time. And after 1,000 rpm the twitch goes away. If anyone has any insight it would really help this annoying problem. Thank You.

I took off the large connector that goes to the gauges in the middle of the cluster, and i found a couple of pins that were green with corrosion... i cleaned those up, and it's like 95% better.