On long trips (45 min on the highway or more) my car starts to misfire under load at low rpms. Any other time, the car runs great. The car has 103k on it, new plugs, wires, coil packs, and a MSD DIS-4 box. Any ideas of what the problem could be?

I've heard people having probs with that DIS-4 box. Is there any type of "diagnostics" on it?

This problem started happening before I got the DIS-4, and as far as diagnostics go, I dont believe it has that capability.....