Ren,
Remember what you are seeing is the coolant temp, not the
oil temp. In most cases, the
oil temp will run about 210-220 F. It sounds like you have a flow problem, either water or air. At hwy speed, you are getting a ram air effect, and even though the temp is 190, it does not look too bad, maybe 5-7 degres high. But as soon as you lose the ram air effect, the temp comes up. 210 is too high. The coolant temp you see on your gauge should be fairly close to the thermo's opening point, and it should stay there unless there is an excessive amount of idling or the ambient temp is high, I know, I live in Houston.
Removing the thermo may lower both the hwy and slow driving temps, but you are really just masking the real problem. It will cause the temp to rise slower, but may not let the engine reach a good operating temp. The
oil needs to be hot to drive off lighter ends like fuel dilution.
Also, check your timing. If it is too advanced the engine will run hot since it fires early and the charge gets compressed further after ignition, increasing the cylinder temp. Also a lean fuel condition will cause an engine to overheat. The gas actually contributes to combustion chamber temps, maintaining proper cylinder temps with some cooling. Too much air in the charge and you lose the cooling factor.
Sorry to ramble, but air and water flow really sound like the problem based on when you see the temp rise and lower.
One other thing. Check the coolant concentration. Too much
antifreeze can be part of the problem also. But don't run just water, you need some corrosion inhibition.
Jerry