Here's an intersting thread about our boost solenoids (pressure converter). I had always thought when these started to go bad, the indication would just be low/under boost. But looking at the graph in this thread, it's clear there is also a failure mode where the armature can get sticky, and they can pass too much vacuum at low wgdc% - which could cause over-boost (the green curve is the bad one). So a bad boost solenoid can cause a range of over or under boost problems, depending what's wrong with it.
https://www.e90post.com/forums/showthread.php?t=1589166