2001 audi a6 quattro 2.7t coolant issue
 

so i am at my wits end at this point, ive replaced the radiator, water pump, thermostat, hoses for everything, reservoir coolant sensor temperature sensor think i replaced one more im forgetting and its still overheating i have bled the system...countless times i have used every type of coolant that can be sold (and tasted because of accidents) its not the head gaskets i had those checked, but for some reason its still overheating....and not like actually overheating i dont let it it just gets to right below the H then i shut off the engine. the coolant isn't leaking except for the times i spill it, the only thing i can think of is the little plug beneath the reservoir is broke...it looks like it still works but i dont even know what its for i am thinking its for the level sensor....also how much coolant does it take i wanna rule out if im not putting enough in......any ideas anyone??