I just picked up my 1985 300D (California) this summer and it has been great so far. Over the summer it was running a steady 85c-95c depending on A/C usage and traffic didn't seem to matter how hot it was outside.
Winter has now set in down in Texas and I can't get the temp up to 80 even with the A/C blasting. My normal commute is 15 miles each way and it now warms up to about 70c and stays there even on longer drives.
Everything else seems to be in order all fluids and filters are fine.

Is this normal behavior in winters or might there be something wrong?