why does my Renix runs cooler all over sudden?

i did the water pump and Tstat 6 months ago, ran great til about 3 days ago.. now the temp gauge only shows 1/4 of the dial ( used to run at 210 on the dial)

it eventually gets to the normal operating temp ( after 1 hour of driving around town ) but then when i get back on the freeway the temp goes down to 150 or so...

do u guys think my TSTAT is defective ( autozone brand )???

i also noticed that the heat is very weak even on high setting

thanks

crack the coolant sensor on the head, see if it's got an air bubble working around in there.

If no air, yes to the t stat sucking.