Asicaster

Hi everyone, brand new to the world of Audi but I recently purchased a 1989 Audi 200 with 213,000km on it. It's a really nice car, and practically in perfect condition. Some lines and calipers needed replacing but overall it's great. ONE issue (after having it plated for about two weeks) is occasional overheating. It has happened twice while driving the car in the city with medium traffic. I believe both times I was NOT running the heat for the cabin. The overheating does not seem to happen when I run the cabin heaters (which I hope means it's not a serious issue). The only gauge that does not work on the dash is the engine temp, yet the "ok" light does show the overheating signal when it occurs. The overheating symbol was accompanied by quite a lot of steam bursting out around the anti-freeze reservoir cap. I made the mistake of backing the cap off which seemed to make things worse, and I have since learned that lowering the pressure decreases the boiling temperature which just made the boiling worse. I have some mechanical experience so my theories are these, but I would of course love some insight from some more experienced people here. I am going to test some terminals now but thought I'd reach out for advice in the meantime.

1. I believe I have not seen the large radiator fan working. I don't know if the fan has failed itself, or if the switch that activates it has failed.
2. I suppose the thermostat could be stuck not allowing coolant to flow properly?
3. Are the dashboard switch and radiator fan and thermostat all synced up by any one sensor? Basically is it possible that one sensor is responsible for the bad dash display and the radiator fan not working? Is that sensor the thermostat?

Thanks for reading and please help if you can!