Kevin;
Good info from 142G! If problem started only after "service", it's likely they mucked something up in either the servicing or simply the refilling of Cooling system!...and I agree with him about the "run without a TStat recommendation"!
Questions: Was Cooling Sys refilled with equal amount that was drained?...was CS burped of air at refilling?...was replacement TStat a bypass type?...what was temp rating?
Simple as they are, the vintage Volvo (clearly with a retrofitted) sealed CS with overflow does have a specific fill procedure and things to watch for when servicing, that a mechanic used to more modern vehicles might not be familiar with...
Suggested reading:
https://www.sw-em.com/Cooling_System.htm
Cheers