i beleive the reason why moving him works is you get the creeper crud off your screen. The only way it seems to avoid it is, yeah, by moving him. Again, the other day i went to help a friend kill moloch and my screen was freezing up before he was even on it tho. After the 5th time in a row i finally gave up and apologized for the lack of help i gave, wished them luck, and left.
I'm about 99% positive it's the creeper crud due to the fact when i use poison creeper on my venom barb in tundra or in big party games everywhere it does the exact same thing. I used creeper actually against the ancients when i duo'd them with someone also. It was very handy, i ran them through the crud repeatedly for some nice dmg (since creeper is lvl 50+ with two carrions and i use a -res +skill dmg setup). However, The lag became so bad i had to take the carrion rings off and cast a random spell to unsummon him.
I understand why the crud is there, it lets you see which area is going to infect monsters. But perhaps there's a way of marking/not marking it with a different or no image? Sounds like complicated programming, i dunno but would solve some issues imo if it is possible. Moloch's lag seems to be his best attack, it quite often causes your death and also at times leaves him invincible. It's also very annoying to wonder how bad the lag will be when you cast poison creeper.
Edit-
My computer is not the best btw. I know others have issues with lag at moloch, but any rabies builds out there have the same issues with their creeper? Maybe it's just me?