can anyone help? after 30 minutes of driving doesnt matter if on the highway or not the motor a 4.2 v6 1987 automatic 3 speed just shuts off, but only when coming to a stop. Turn the key all the way off then restart and it will run but with a rough idle wanting to stall in mid travel on or off the highway but now whenever you come to a stop, idle under 800 rpms. Found though if you put it in neutral and rev the motor between 1200 to 3000 it wil continue running.

any hints or ideas please reply

 

Try cleaning the idle air control valve