Hello everyone,  Thank you very much to everyone who had suggestions
and ideas for this pesky problem!  The good news is that I fixed it
today. :-)

After trying a number of things, in the end the solution was simple
-- I glued hammer rail felt to the inside of the spill vent hole,
all around the circumference.  This does not clog the vent but just
provides a softer outer edge of the hole and it also makes the hole
a little bit smaller.

The regulator will now only oscillate 2-3 times maximum if it is really
aggressively provoked, and then it will dampen and settle down.

Kind regards,
Bernt Damm
Sydney