I tried your suggestions. Thanks. On my machine the CPU frequency was fixed at 4.00GHz and it wasn't obvious that it could be changed anyway. Similarly the power limit was already set for 88w, but I disabled Intel Turbo Boost. The Core Limit was already set for 105, and I disabled the K OC. I set vcore to 1.19.
This seemed to make a small difference, but I don't regard the result as satisfactory. If I now run Prime grid using 45 % of CPU time I get temperatures varying between 54C and 92C, with temperatures in the 80sC for quite a lot of the time. I don't think it's a solution for me. I'd expect to be able to run Boinc at 75% of CPU time without problems.