my 1987 300DT runs hot too. regardless of ambient temperature. i don't think it ever overheated, but the previous owner went thru TWO radiators one month apart, two years ago each time with a tow charge.

why do I mention this? My RADIATOR is already CLOGGED! The passenger side is "ice" cold and the drivers side is HOT. Somebody elsewhere said it was the DEXCOOL introduced by the previous owner after the fact that turns into brown sewage. That describes what I see in my upper radiator connection at the head.

I am saying this, check your radiator even though it was just replaced, to make sure there are no hot/cold spots.

Dont' know if that will help, but its the best I can suggest. good luck,

Brian