1stly hi

heres the story ,i bought the wife a 01 a4 1.8t quatrro a ear or so agao ,when we bought it the heater wasnt brilliant so i changed the thermostat which made the heater slightly better ,she drove it for a few months then 1 day it started boiling up and getting hot i checked the coolant reservoir and it had oil mixed with the water ,so i thought head gasket ,pulled the head had it crack tested and skimmed put it back together same thing oil in the water i did some research and found out about the oil cooler problem ,new oil cooler solved it ,drove the car for another month or so and the check engine light came on and it started running on 3 plus it was still pumping the water out so i thought enough was enough ,head off again ,again rechecked ,new timing belt ,thermostat ,water pump ,tensioner ,coil pack and put it all back together still pumping water out ,new coolant reservoir solved that ,again drove it for a few weeks and the new reservoir started leaking and it got hot ,got a new reservoir today put it on bled the system by pulling back the heater hose and elevating the coolant tank ,both with it running and not ,the heater is crap unless the car is running at 2500rpm then its nice ,had the car running in the shop for a good hour ,temp normal and heater hot took the car for a test drive 2 miles down the road it starts to get hot ,not leaks anywhere when i get it back to the shop its pumping water out of the overflow of the coolant tank ,i let it cool down and refilled it and re bled it but it did exactly the same once i drove it ,i pulled the thermostat out and checked it in hot water and it opened fine so im stumped never had this much trouble with any car lol i flushed the rad ,heater core and the motor with water and it all flowed thro fine so if any1s got any suggestions please help

sorry for the long post