idle problem

[hank]

have had this nagging problem for the past week or so...

truck starts fine and runs fine but once a day or so it starts to idle really rough at stoplights. it runs fine when i put my foot into it a bit, but returns to running roughly when i stop again. if i kill the truck and restart, the problem goes away.

my truck is a 71 cheyenne. it has a 350, all stock except for HEI. just changed my spark plugs/wires 3 months ago when i switched to HEI.

any thoughts or help are appreciated.