intermittant smoke in exhaust :(

[quiksilvia]

so if im at a stoplight or while my turbo timer is running, my car will sometimes decide to let out A LOT of white smoke-coolant-not tight. but my car has to be idling for at least about 30 seconds before the big cloud comes out. and its only like once or twice a day and i drive a lot. i just swapped my head in october with a new metal hg and i have no other probs. could it be a bad ring? tia