Hi Bob,
I would really have to see a short movie of that happening to let you know whats going on. I am almost certain that it is intentional (yet unexpected by you) behaviour - we have had thousands of people operate the aircraft for well over a year and this is the first time I heard anyone have that problem.
There are several cases where the speedcursor will move seeminly "on its own" - what you describe sounds like its a "runaway" problem where you grab the cursor, and then loose view of it (the sim considers that as holding and not letting go) and then move the mouse wildly, maybe trying to manipulate something else - but really just moving the speed cursor "to the stops".
Again, a video would say more than a 1000 words, any chance of capturing a short one for me?
Cheers, Jan