ZR2 Tach Problems HELP!!!!!!!!!!!

I have a 98 ZR2 4.3 TBI truck.My tach is doing some funky stuff and has now got worse.While driving down the road the tach will just start to jump up and down,while it does this the engine will run rough.This jumping tach only lasts for maybe 5 seconds.The tach then goes to zero and stays there.The truck runs fine once the tach dies.Once truck is shut off and left alone for a few minutes and started again the tach works again and so the process starts over again. :screwy: I have done a visual check on the wires under the hood and everybody looks good.Other than that,I have no clue.HELP!!!!!!!!

Check the connector out on the coil for corrosion, backed out terminals, bent terminals and do the same for the one on the ignition control module

Where is the ignition control module?Is it on the cap?

Checked them and they look fine.