I'm having a problem with my tach. At idle the needle swings at random from idle to 4K. I have an autometer tach as well and it does the same thing at the same time as the factory tach. The tach moves at random and just bounces. It doesn't stay at a particular rpm just swings back an forth then goes back to normal. I also have a seperate shift light that I think is working correctly. Sometimes the tach works just fine. I don't think the engine is missing or is loosing any power. Its hard to stage the car when the tach is bouncing all around.

 

check your connections.....

 

Double-check your grounds. I had a similar problem once and found a ground wire not securely connected.