Not so much with the ARs, but I find that a couple of my pistols run like absolute garbage when they're perfectly clean. The Antichrist (Glock 22) is especially bad about that, actually. Hell, I've even heard of some people recommending to just use 10W30 on them.
I tend to find cleaning the boomsticks cathartic, so I typically just take the time to use Break Free and elbow grease, with the female end of a cleaning rod and bore/chamber brushes for the afore mentioned trouble areas. If it's really bad, I may use just bit of carb cleaner. For the barrel, I've also used some of the foaming solver stuff (can't remember who makes it) with good results.
I should also note that since my wife saw the insurance rider for the firearms, she actually pesters me to make sure they're clean. Which is nice, because it gives me an out from the estrogen fueled hurricane of destruction that is my house.