Load,
Tracked this situation for many years. Exact same symptoms as you describe.
My solution was a 195 deg. t'stat. [test in boiling water first!]
It will maintain 190 or so when highway driven with much less range or fluctuation in water temps and it won't overheat in warm weather.
Oil cooler blocking seldom works when ambient air is 50 deg or less. I've found my Mocal
oil t'stat to be of little help.
Have found a wide variation in gauge accuracy. Try to use a laser-type temp gun for much better accuracy and calibrate your gauge from that.