The pressure gauge definitely is boiler pressure. The only relationship between flushing water through the HX and the boiler pressure I can think of is that it would lower the boiler temperature slightly. That, in turn, would reduce boiler pressure a little and might trigger the pstat.
My best guess is a defective pstat. It sounds like you have the equivalent of a really wide deadband. Check and see if you have a CEME or Mater - the Bricoletta did not use a Sirai. The CEME has a deadband adjustment, which normally should not be adjusted by the owner. If it is a CEME, you might be able to adjust the deadband and fix the problem. Otherwise, I would just replace the pstat and see if it fixes your problem. While the symptoms you describe do not clearly scream "pstat," I can't think of much else it could be. The pressure stat turns the boiler element on and off, and it is not turning it on when the pressure drops below the desired level.
My Bricoletta has a deadband of about 0.4 bar. Anything much larger would be off. Your initial readings of 1.7 or so would be too high anyway - a range from about 1.1-1.4 would be about right, i.e. the pstat should turn the boiler element on at 1.1 and off around 1.5 bar.
I cannot remember whether my Bricoletta uses a Mater or CEME - if you post a picture of yours, people here on H-B can easily identify it and tell you which screw adjusts the setpoint and which adjusts the deadband. It is quite easy to break the pstat by trying to adjust the deadband on a CEME, but if your pstat is malfunctioning, you may not have much to lose.