UPDATE: After a couple more attempts we seem to have sorted it out. It's been 2 months now without recurrence of the problem. We noticed that the piston, which drives the cup sensor, was VERY slightly off-center. Apparently the housing/sleeve that the piston moves through, was accidentally rotated during a previous service. The piston was thus forced to move slightly off-center. Looks like it contacted the cup at a slight angle, causing a spring to stick and not allow the cup to move as required. The rotation apparently happened a year ago, so why this only became a problem now, mystifies me.
But apparently the spring sticking happened on at least one other instrument; in that case they bent the spring in a specific manner until it started working again.
Deon.