Similar problem solved on TAMD41P-A
I had a similar problem - RPM past 3000 drove the temp above the 191 degree specs, so I backed off. My excellent mechanic checked out the exhaust system and found a clog of heavy, wet black stuff partially blocking the system. He dug it out, replaced that hose.
Next time out I ran the RPM up slowly, reaching WOT in specs - about 3775 - and the temp stayed under 190 and when slowing to 7kt the temp fell very quickly to 165. I ran several segments of WOT of 15 min or more that week, and had the same outcome, SOG both ways at 14.8kt at WOT. I suspect too many hours below 3000 RPM for years with no RPM close to WOT are at fault (i.e., ...."My Fault".)
Aside from that problem and fix, I am completely pleased with the Volvo engine and transmission. Runs great, especially with consecutive fuel loads of ValveTect diesel - silky smooth.
This may not be the same problem as with your engine, but this was the problem and solution for mine.
Passagemaker contributor Steve Zimmerman has written several articles about engine RPM, % of effort, fuel burn, engine life, etc., all worth researching. I've been planning to take his diesel mechanics course at Trawlerfest, but COVID got in the way. The class I took from him on selecting a boat, etc., was outstanding - 2016, Kents Island Trawlerfest. We'll take the Boaters' University series this fall including a diesel mechanics course.
Best wishes for an inexpensive and effective solution to your engine problem!