It took quite a while, but my second PC finally got fixed. Goris located the problem some weeks ago: the processor cache was faulty. However, getting a replacement was quite an issue. The mobo only takes older AMD processors and Goris didn’t stock them any more.
I had to track down a store that sold the right processors at the right price. And I needed one that would fit. The list of valid processors is long, but finding one that’s priced correctly, reasonably fast and available was quite a hassle.
I finally did that last Thursday, in Rotterdam. On Friday, I went back to Goris and yesterday, I picked up this fine machine of mine. And these lovely folks at Goris didn’t even charge me for the effor they themselves put in!