Anyone experienced anything similar?
I had a problem with the vibrate, not random just couldn't turn it off, tried; settings/sound/advanced/, then scroll down and turn off 'touch vibration', but still had the vibration, from memory only on some but not all input, maybe just some keyboard or touchscreen input, sorry I can't fully recall.
After searching I found another option; settings/system/languages & input/virtual keyboard/planet keyboard/preferences/, then turn off 'vibrate on keypress', which fixed it for me. I now only get vibration on notifications when I want them.
I'm on V21 now, but I found this on an earlier version of firmware. I've routinely performed the same two checks after updates and a couple of flashes, but I haven't seen any difference in behaviour across firmware versions.
I can't recall any more specifics than that, but yours definitely sounds a bit odd, mine was consistently annoying rather than randomly annoying, and I never saw the thing with the key combinations you describe.
Try the two settings above if you haven't already found both of them, but failing that you may need to flick an email at Planet for support.