I just had the radiator/thermostat replaced (Dec.2000) in my 84 300DT and experience the same simptons. Prior to change out temp always just a nudge above 80C, now is runs just a nudge below the 100C. Another curious thing, the pressure escaping when opening the expansion cap is dramatically less than before. The old radiator leaked at the top tank, thus the need for replacement, would this leak cause it to build more pressure in the system? Counterintuitive I know but it just seems odd.
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)