I have a 2004 a4 USP quattro with 175k on the body and 30k ok a built block. Yesterday I got a coolant flash check on my dash. I checked it and it was full. I started my car again and now my heat is blowing straight cold air. It takes a good 20 mins or so driving to get a little bit of hot air. This morning I'm on my way to work and my coolant guage goes to 3/4 and then all the way on red H. So I thought it was over heating and about to pull over and suddenly it went back to normal. And now I have blueish/ white smoke out the exhaust pipe.

Any help would be appreciated. ..was thinking HG...turbo possibly. ..thermostat. ..but now I'm lost. ...thanks in advance