318 overheating

So I just got a 1971 Dodge Dart 2 years ago and have been working on it and driving it for some time. Last week when I was driving my temp gage went dead. Ok I’ll go work on it this weekend. Today I started the car up and the temp gage was working. Ok cool. Started driving it and the needle was between the e and m on the gage. Then after a bit it climbed to the M. So I got where I was going and let it sit for 3 hours. Came out and it was reading at the T (like 1/4) I thought it was odd but whatever, went to get some food and the gage hit the line before red. So I got back (it was 1/4 mile drive and 2 mins at a drive through). It spit out prob 3oz of coolant, I went to see if I could borrow a temp gun from my coworker. Upper rad hose was 207 lower was 198, intake was 210. Rad was 200. When I got the car there was no thermostat in the vehicle so I put a 195 one in. Drove it a few times and it was all fine. So I had to drive this home but after 3 hours of sitting, the temp gage is reading 3/4 hot. So I start driving and it sits at the line before overheating, doesn’t go over it, just sits there. I turn on the heater and the heat isn’t insanely hot like I would think. So I am driving it and after about an hour it starts to go into the hot zone. Pull over. Open the radiator cap. Coolant is full, and while idling I see the coolant moving. Sit here for 30 mins and get finally home. So it’s burping a little coolant and I get a meat thermometer and put it in the coolant on top of the rad. 214. Gage is saying it’s overheating. I don’t hear anything from the engine like knocking or position slap, not even an odd misfire. Last week I put a new temp sensor in so I figured I’ll put the old one in. So I do that, fill the coolant up and. Start the car. Temp starts rising up past 3/4. I don’t see anything swirling in the radiator like no movement of coolant. The lower radiator hose is not collapsed and is cooler than the upper one. When I turn it off and put the cap on. I can feel burping in the upper rad hose. I’m at a loss to what to diag next. Could my new thermostat not open up at the right time anymore? What should I do next?