first of all i apologize for the newb question.
ok so i have an 06 sti and its my first turbo vehicle, while i was driving down the road i was in fifth gear and the temp gauge went to about 3/4 heat and i shifted down to fourth going 50mph..and it went down right away. also i was sitting at the shop just idling and the temp went way up again so i drove away and it went down again..i have no idea is it a thermostat, bad antifreeze? what? if anyone can help me please do cuz i dont wanna blow it up. and as info it has a bigger turbo,injectors, fmic. intake all that but hasnt been cobb tuned..just bought that comes tomorrow. please help!!