just purchased a 1998 hb 1.6 dy7 na only mods are full headers and exhaust and cai car has 150,000 miles and just started to over heat did a complete flush of cooling system checked thermostat and temp sensor both are new also bought new rad cap. cooling fan does work hard wired it to batt just to make sure. problem is there are no leaking hoses everything from radiator is blowing into overflow bottle and boiling it out. also the temp only rise when sitting and a little after i start moving temp goes down i know air through rad is cooling temp lil more but the thing that gets me is the heat inside car changes from hot to cold when driving and only cold when sitting. i have searched the site but have found nothing like this. could it be the head gasket leaking gasses into cooling system ? checked oil and no mixing of coolant and oil and no big puffs of white smoke . could water pump not be cycling coolant enough ? also when i start car after sitting for over an hour the car starts fine then a min later its sputters acts like its gonna stall out the for maybe a min the runs perfectly fine no cel did fuel injector cleaner and sea foam. sorry for the long story but im stumped please help.