Three weeks ago I bought used 1986 Volvo 740GLE with 135,000 miles on it and I liked the car from the first day. But…

Two weeks after I bought it, during highway ride my temperature gauge suddenly nailed pin in RED area and coolant escapes through tank cap spraying over engine causing a lot of white smoke. I stopped the car immediately aside of highway and opened the hood.
Suspecting faulty thermostat I opened its housing and found thermostat literally broken on a three spots. I called my brother with an instruction and he brought me a new 87’C thermostat and coolant. After I replaced thermostat, I continued planned 100 miles trip with no further problem.

Now, I have noticed this:

Only one minute after start the car, temperature gauge is already showing that the car have reached normal working temperature, positioned in the middle of the range, where it stays all the time after.
For test, I stopped the car as soon as gauge stopped in the middle of the temperature scale and touched engine. Intake manifold was almost cold, valve cover just started worming, so it seemed to me that the gauge was not showing real temperature.

Can somebody help?