Are you on a mac?
If so, grab a copy of Snoize MIDI monitor:
http://www.snoize.com/MIDIMonitor/
...and see if you can figure out where the random panning messages are coming from. I have had keyboards that output pitch wheel info randomly before, so what you're describing certainly sounds like it could be from the keyboard, but the only way to know for sure is to see what's actually going on.