i do 20 min of stop and go and it runs fine, but i still think its a small ceck on the HG. Last time my headgasket went out it ran like straight up ----. lots con condesation in the oil, tons of white smoke and would over heat as soon as i got off the freeway.

 

do a leak down test yet?

 

Its a headgasket. a simply 60 dollar fix + time. Happens all the time on D series, i'm assuming thats what you have.