Electrical or vacuum problem?

My '88 Accord LX runs improperly between 2000-3000 RPM under load. Typically this is in lower gears when I am accelerating. However, now that it is warmer and I am using the A/C more, it does it on the freeway in 5th gear under additional load of hills, etc. I am not sure if this is an electrical (i.e.distributor - wires, plugs recently changed) or a vacuum problem or other.

This problem developed after I did the timing belt about 11,000 miles ago. Fuel economy is excellent, 34-35 MPG on the freeway at 80 MPH.

Thanks,