My 2004 C2500 generally runs great (other than the leak I discovered at the heater core T connectors today!). The only odd thing is that at a light, while in drive and foot on the brake and the truck is idling, every 15-30 seconds there is a slight "stumble", maybe its a miss? It is not 100% consistent in that its on a cycle or anything, but it happens several times per minute. Its enough to feel a quick single vibration through the seat, but not enough that the RPM meter drops, its that fast. No CEL illuminated. Plugs are new and properly gapped. Under throttle the truck drives just fine with no power issues or temp issues - runs spot on center on the temp gauge. Not a major issue just an underlying concern I have. Any thoughts on what it could be?

 

New plugs AC ones? I had idle issues with NGK and went back to AC plugs. Lesson learned.
Use the GM T connectors when fixing the leak. Aftermarket ones are junk and don't seal right. I changed all the hoses that plug into the T's on my Tahoe along with the T connectors.
My leaking connectors also were dripping on the back ignition coil, that may be your problem also.