I, too, have experienced this problem and, if I remember correctly, the brew pressure dropped around 1.0 bar. More dramatic drops were reported in the same thread as they are in this one. It was and is baffling.
My problem was "solved" by thoroughly descaling the OPV (removed from machine) but I could not then and cannot now "connect the dots" as to how this solved the problem.

I now believe it is caused by an intermittent electrical problem INTERNAL to the Ulka pump - that being an intermittent failure of the diode which functions as an electrical check valve. I would not think that a diode could have an intermittent failure but, after all, this is an espresso machine -

I initially assumed that all of the backflushing I did during HX de-scaling cleaned out the OPV and solved the problem, but each backflush pulled in more de-scaling solution through the vibe pump, so maybe it was the descaling of the vibe pump that did the trick (?). Or not.
-Ed

I'm still working on diagnosing my Brewtus II. At this point the steam boiler refill keeps calling for water; so there is zero flow going through my machine.

Before getting a new pump, I want to be sure that the water path is completely clear throughout. Up here in Canada a local supplier quoted $125 for a new Ulka pump (compared to $50 in the US). Now I need to find a US parts supplier willing to ship USPS (so that I don't have to pay an equivalent amount of the pump in brokerage costs).

Mark

Stefano's Espresso Care ships to Canada

Thanks Ian. I appreciate the suggestion, and will certainly order from Stefano's if I buy a new pump.

Mark