1985 Chevrolet Chevette Hello Having Problems Again Whenever Sit Traffic More

Hello. My Chevette is having problems again. Whenever I sit in traffic for more than 5 minutes, my car starts to have a rough idle. It's progressive (it starts shaking a little, then it kind of misses and if I wait too long it nearly stalls). As soon as I drive off at normal speeds for a few seconds it goes back to normal. I changed the coil, the spark plugs, the wires and the distributor cap and the problem is still there. Any idea?

Thanks.