When did it start? Here's a possible scenario: vacuum leak on 2 & 3 (two separate leaks). To compensate, you adjust the idle mixture richer, which smooths out the idle, but now 1 & 4 are way overrich (2 & 3 are happy though).

Or maybe it's an ignition problem. Check the distributor cap throughly for tracking, and check the cam lobe to ensure the points are opening the same amount on all four lobes.