1995 Oldsmobile Aurora

I have had this car for about 8 months. It has had various parts replaced nothing major, alternator, radiator, belts and hoses, etc. I was driving it a few days ago about a week after replacing the alternator and the tachometer would jump like it wasnt getting an accurate reading for a split second. It did this throughout the day. Then I went to the store everything was fine came out to leave the store and it wouldn't start. It doesn't crank and the fuel pump isn't running when you turn the key. You can hear all the relays clicking like it is supposed to when the car is started and I checked all the fuses. The security light doesn't stay on but the message center display and the outside temp display both flash when the key is in the on position. It has only done this when connected to a code reader. I have replaced the computer with a known good unit and nothing changes.