No i have not scanned the computer ill give that a try and see what comes up. thanks for the advise

 

Definitely check for any OBD codes, and check the new IAC. Take off the air hat and with the truck running look down in the port where the IAC pintle is and adjust the throttle and see if the pintle moves out and back in. It is a little stepper motor so it will make a jerky motion as it opens and closes. I replaced mine a few years ago and the new one was defective.

 

How much of an interval between the smooth and rough idle? Is it almost a consistent offbeat jiggle while at idle and does it smooth out fine upon acceleration? Sounds a lot like issues I've been having and all things I've tried, besides 01 sensors(denso) and new TPS from dealer. Some have suggested cracked valve seats.
Have you done compression test? Make sure to follow up if you figure it out. Good luck.

 

i think we are experiencing the same thing. yes like a rough shake or jiggle. its a few seconds apart from smooth and rough. no have not done compression test. How would we check for cracked seats other than visually?

 

Cylinder leakdown test. Will tell you where compression is escaping. (you can hear it.)

 

So i was messing around again and tried to cover the butterflies to see if it would choke the engine and nothing happened. Didn't even lower rpm's. This means air is being drawn in from somewhere else right? I dont know where though the plenum is brand new along with throttle body gasket. any ideas?

 

IAC allows air in when butterflies are closed at idle, other wise engine would stall at every stop sign. And yes it does sound like the same issue. Jiggle,shake, jitter and every couple seconds a little harder shake with a 50 to 75 rpm drop. Most people would just call it normal for magnum engine, but I've had couple other trucks to compare to. My issues were worse when it's warmer outside. The plenum repair and timing set made an improvement, but not good enough. I'm going to get a leakdown test soon. It's really annoying in stop n go traffic.

 

Ya im going to chang emy plugs back to what was in there when i bought it and do compression test in the process. im going to do it this weekend ill keep you posted. And yes really annoying even if it is "normal'

 

well i checked compression this morning and all cylinders were good. in the process i took out the ngk plugs and put in some autolite xp 5224 plugs. It helped a lot actually. i would say it normalized it even.

 

What did your ngk plugs look like, I mean the center part, clean and white or light tan? Interesting that iridium plugs made a difference.