Another thing to check is the vacuum advance on the distributor. The timing only gets advanced at idle and when coasting in gear, and non-advanced timing will definitely make a difference at idle. Unfortunately, it's a hundred buck part, even at the cheapest parts houses.

My Truck: 1987 XtraCab DLX 22R 4WD 5 Speed Manual
--------------------
"Speed has never killed anyone. Suddenly becoming stationary, that's what gets you." -Jeremy Clarkson