rough at 1500

i had a bad distributor cap. 2 in 7 months. i replaced the plugs, wires cap and rotor. that fixed the hesitation. again. now at about 1500 rpm's sitting still it idles rough. not real bad but enough to annoy you. runs great and at regular idle it's perfect. i was told the carb might be dirty. oh i also changed the fuel filter. i put some stp complete fuel treatment in her. could the carb being dirty be the cause or could it be an underlying problem?

What year is this? The trucks in this section dont have individual coils and are fuel injected.

I would check the timing before you start messing with the carb adjustments because when you changec out the ignition parts it ran good.