stumbling/misfiring at 2500-3000 rpms

[60ratrod]

ok so lately, as in the past few days, my truck(94 gmc stepside 4.3l auto) has been stumbling/misfiring at around 2500-3000 rpms(i can't tell exactly since i don't have a tach) while going down the highway. it only happens when i'm cruising, trying to maintain a steady speed. otherwise during any kind of acceleration, like gentle part throttle/hard wot acceleration she doesn't stumble, just when i'm cruising trying to maintain speed from 45mph-55mph. any ideas?? i checked the cap and rotor, i don't have any visible fuel leaks, i have spark on all 6 cylinders, only pulled one plug cause the motor was still too warm to attempt to pull off any of the other wires and it looked alright. the only mods that i have done are an intake, tbi spacer, msd coil, accel tune up kit(plugs wires cap rotor). could it be a ign module in the dizzy?? oh and the motor has about 145xxx so far as i can tell. oil was changed at 143,143(exactly), along with tranny fluid, filter, trans pan gasket, trans tail housing seal and bushing