The problem has been solved: Engine was simply running too lean. It took a lot of 'trying things', including suggestions from this forum.
It's a rather interesting story how I ran into this problem in the first place. I bought the car with dual webbers on it, but decided to convert to a couple of SU HS4s, as the webbers were way too rich for my type of driving, and the result was usually fouled plugs once in a while.
After I converted to SUs, the run on problems started. When that happened I asked a british car mechanic about how to trouble shoot this problem, and his suggestion was to try a cooler plug, which I did. The runon stopped for a while, but I also noticed that the plugs were way too black. I gradually adjusted mixture to get the right plug color over time. What also happend was that the run on problem reappeared occasionally, which then led me to re-install the anti-run, which had been removed by the previous owner when he installed the webbers. I also added an oil cooler. Those two actions led to some improvement, but didn't fix the problem completely.
All along I had no idea that I was running lean, other than that I had to back off on the distributer advance somewhat to prevent pinging under load. Early fall I had an opportunity to put my car on a dyno to see what hp and torque it would show. It wasn't as good as I had expected. The technician pointed out that the engine was running lean. We adjusted the mixture a full turn, and HP came up. When I turned the engine off at home, guess what, no more run on. I drove the car for a few more days without doing any further fine tuning, and then I had a look at the plugs, and....they were black again.........All of a sudden a light came on. I had sent myself down the garden path by putting in cooler plugs. Went out the next day, bought the plugs the manual calls for, and haven't had a run on problem since.