white smoke ?

i have a vectre the problem is i have white smoke now this only happens after a long run or when i try to over take and it happens in gears of second to 5th gear the car will stutter then i get a big puff of white smoke its that bad that you can not see the road behind you i have found if i pull over into a layby switch off and lock the car for 5mins then unlock the car start the engine it sorts it out i have tried stop smoke and redex but still have the problem i am not losing any water at all and not burning oil and there is no water in in the oil so i am guessing the head gasket is fine can anyone tell me what is causing this problem thank you for your time and help