Unusual warm up condition

[Histoy56]

Last year I purchased a '56 BelAir with a new 350 crate engine (290hp)with a chrome Edelbrock EPS intake. It has a Griffin high capacity aluminum radiator and the original 4 blade fan. Last year because of Covid, our club limited our activities to birthday and Senior Care facilities drive bys. In essence they were just slow parades. My engine didn't like those and had a tendency to run hot. I switched from a 195 thermostat to a 160 one, but that didn't help much. I then installed an electric fan in front of the radiator, which helped some. Over the winter I installed a 17", 6 blade fan so the engine should run cooler this summer. I do have an unusual warm up condition. I've installed 3 different 160 degree thermostats that have all intermittently given the same warm upcondition. When I install them I run them in my garage and watch the temp gauge to see when the stat opens. They all open around 160 degrees and hold that temp. When I drive around town, sometimes they open correctly, and sometimes they don't. When they don't, the temp goes to around 200 before the stat opens, and then the temp drops to 160 pretty quickly. I had the car on the highway a couple of weeks ago and the temp went to 230 before the start opened, then it cooled to 160 pretty quickly. That's when I bought the 3rd one, a never fail one. We had the car out to a cruise-in on Wed. and the stat worked fine. Today I took it to another cruise-in and I noticed that the temp went up to 195, so I opened the water control valve to the heater and the temp started dropping immediately. I closed it before we headed home, and I watched the temp and as soon as it hit 180, I opened the water control valve to the heater again, and the temp dropped immediately. The water flow from the heater enters the intake manifold right next to the thermostat housing. I'm beginning to think that there might be an air trap or something in that location that is causing my problem. I always drill a 1/8" hole on the thermostat, which I think should eliminate an air trap, so I'm confused. What am I missing?