If it was running cool before you changed the thermostat there might still be
air in the system. Can be stubborn. Bought my 82 diesel for $1500 with
problems in Santa Barbara 2 years ago, drove home to San Luis Obispo  and
temp never over 1/4. Replaced a clogged radiator, absolutely wornout
waterpump and housing, clogged heater core, and new 82C thermostat. Result?
Just like yours: 1/4 at idle or around town, 3/4 at 65. Keep bleeding, it
seems to take forever. Now it's too cool. I had my stock radiator recored
from 2pass to 3pass for $180 instead of $350 for the one year only 82 new one.