car bogging/misfiring while idling

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I have a gsr block, b16 head crower springs and reteainer, type r pistons and crower cams, camshafts, msd extewrnal coil, p28 and when my car is idling my muffler is popping like if it misfires. also when I am driving steady. it help when i changed the spark plugs and wires because the old spark plugs were new but were not gapped before being installed. and the were really black. when i changed them it helped the motor do you have any idea what it could be???
the wierd thing is that it doesn't do it all the time. it may be some type of sensor that is going out. maybe the map sensor like you said but do you know how to check it by volts. the is no check engine light on and wouldn't it turn on if its the map sensor?