96 GMC Jimmy - Tachometer Erratic - Engine Running Rough

So yesterday the tachometer started acting weird and sitting at 0 or fluctuating between 0 and where it should be (the engine speed seems to stay the same). When I accelerate it the tach seems to jump where it is supposed to be. Today it kept doing the same thing then the truck started running rough when coming to a stop and even died once, but started right up afterwards. I let it idle when I got back to my office and it ran rough then died and again started right up. No warning lights are currently lit. If it was just running rough I might think it was spark plugs or timing. But the tach really has me scratching my head.:banghead:

Any ideas?

I'm guessing the engine computer is probably getting the same mixed up signal as the tachometer...maybe a crankshaft position sensor problem. It should show up in an OBDII scan, maybe as a pending code. You can get a free scan at some chain type auto parts stores.

Tachometer feed is taken from the ignition coil. Direct a mist of water over the ignition coil when the engine is idling, try it when it's dark out. Watch the coil closely for leaking sparks. That could explain all of the symptoms.

could be the distributor shaft mine went bad about a week ago and was doing exactly the same thing i took it to a shop and it was the distrubtor shaft