Your temperatures sound normal except for the t-stat opening temp. The 380 t-stats were notorious for sticking. We used to drill small holes in the valve and cut of the temp feeler. The point was to get the t-stat bulb to heat-up quickly enough to avoid the spike temp.

I would replace the thermostat.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician