90 E250 wont start hot

1990 E250 wont start after been running and hot. it must cool sometimes for an hour before it will fire again. it turns over but will not start. i have replaced the thermostat. there is rust in the system. is there a sensor or any other ideas ?

Have you done any thing else to it? Check all your battery cables for corosion and clean them all including the connection to the block and the starter. How is your battery? Have you change the timing? A weak starter can cause this also as can to much advance.