4.3 engine. will not start. ran great one day. wouldnt start the next. no warning lights on dash. getting pleny fuel. changed ignition coil and ignition module. just turns over and loud pop in engine, but wont start. the last time it would not start i waited a few days. it then started, ran ok for a week then wouldnt start again after it sat for a week. it then started, ran ok for a few miles, the tach began to jump slightly and miss. i tried to rev the engine, it blubbered then reved a few times, then stalled out and has not restarted since.