Troubleshooting stumble at idle in gear

The ignition control unit is fairly new as well, installed about 12,000 miles ago. I've noticed that the problem gets worse the longer I sit in traffic or drive at low speed. It's as if the underhood temperature is affecting it.

If it was a vacuum leak, wouldn't the idle be rough all the time, in gear or not? Also, there's no stumble on acceleration at all.