WILL NOT FIRE

91 CHEVY CHEYENNE FIRING PROBLEM---PUT NEW ON AND WORK FOR A WEEK--THEN PUT ANOTHER NEW ONE ON AND WILL NOT FIRE. WHAT DIRECTION DO I GO NOW?

THE FIRST ONE I PUT ON LASTED A WEEK---RIDING DOWN ROAD @ 60 MPH SHE SHUT OFF, THEN STARTED UP ABOUT 10 SECONDS LATER---I DID NOT TURN IGNITION OFF---GOT TO MY DR OFFICE, CAME BACK OUT AND SHE STARTED BACK UP RAN FOR 5MIN. AND NEVER FIRED AGAIN---BOUGHT ANOTHER NEW ONE---IT WON, T FIRE.