tachometer issue

I am having trouble with my 1996 STS ..every so often i will start my car and the tach doesnt work.I am also getting a service engine soon with aN engine misfire code.When I turn the car off and restart it the tach works again...I recently changed the plugs and wires, but not the gas filter.That is my next step with the engine misfire.Gas cap is OK.

ANY THOUGHTS TO WHY MY TACH DOESNT ALWAYS WORK...?:banghead:

THANKS IN ADVANCE