[BoxsterSteve]

Not on my Boxster, but my old Forester...I had a sticking calliper that wouldn’t completely release.
I would get these crazy random harmonic vibrations that would come and go; highway speeds were the worst. The entire vehicle would start to shake... subtle at first and gradually becoming violent enough to make me pull over and stop.
Pull off the highway, stop for a bit and start driving again and sometimes the behaviour would repeat itself, but not regularly.
Took the car to my Subaru wrench and he thought I was full of **************** until he test drove it and experienced it for himself.
Yep, a bad caliper that wouldn’t release fully.
Caused a goofy random harmonic vibration through the right front/left rear brake circuit that would cause both calipers on that circuit to randomly chatter.
A new caliper and pads on the right front wheel and new pads on the left rear and all returned to normal.