If the temps go down at cruise, it is generally reasonable to assume it is related to airflow ... i.e. not likely head gasket or it would be a problem all the time, right?
Lack of air flow at slow speeds (particularly in low ambient temps) points to vclutch on fan not operating properly ... assuming all else is OK. At that kind of ambient temp, the efan should almost never even get an opportunity to meet the temp limit for operation.
I guess there is some potential that head gasket was badly done and the (1) exhaust gases are leaking into the coolant (can tell with instrument to detect hydrocarbons, I think ... can't remember what it is called), (2) some coolant passages were blocked by the gasket itself.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg