Have a 95 4wd 4 dr with 4.3... When temp in mirror hits 83-84 outside, engine shuts down... will start right back up but will shut down again and again until allowed to cool back down...this happens during stop and go driving (delivering mail)... if driving at speed, it never shuts down...has new crank sensor, dist cap, coil, ignition, fuel pump..... HELP please!!!