Head gasket?

i always have trouble with air getting trapped in the system if i don't fill the engine itself with coolant through the upper hose first. if there's just enough air in there it can look like its overheating while idling sometimes when its probably not.

also, if there's that much gunk in your overflow then there's at least that much gunk in the rest of the system. i just replaced a motor with bad hg's in one of my subes - grey/black gunk all over. i'm running straight water (fill with water, drive for a day, drain and repeat) to rinse it, then i'll use a coolant system cleaner, then i'll actually put good antifreeze ($) in there. only thing is, if it freezes where you live - drain it as soon as it cools off so you don't forget and wake up to a cracked block...