Hot 170

Hi, I'm new to this forum, and also to slants. I have a 63 Dart 170 that was running fine, yet a little hot for my liking. One day while driving for 15 minutes on highway, it got too hot and i pulled it over for a half hour to cool off then drove home. Later in the week we replaced upper and lower rad hoses, added new 160 temp t-stat (was 180 temp) and new t-stat housing. Put it all back together and drove for 20 minutes with much lower temps. All seemed great until I got into stop-and-go traffic. Temp went up-and-up and I nursed it home again. Today I drove her for 5 minutes tops, parked car and let it run for 2-3 minutes and the temp spiked!! Soooo I pushed her home, took out t- stat figuring it was fried but it's not. Help me please before I accidentally crack a block or head or something.