Strange scrollbar problem: lost mouse-up click

[Dan Kegel <dank kegel com>]

For some time now, on both of my computers (Ubuntu 8.10 and 8.04),
and on both Firefox and Chrome, I've been noticing lost mouse-up events when
I click on the scrollbar away from the thumb.  That's supposed to scroll by
one page, but what I observe occasionally instead is either
- the thumb sticks to the mouse and moves with it until I click again, or
- the thumb scrolls very quickly to where the mouse is, covering many pages

Both of these abnormal results are very distracting.  They're not especially
frequent, but they happen several times per session, and occasionally in
quick succession.

Has anybody seen anything similar?

(Both systems have a 3M ergonomic USB mouse, but those have been
working well for years, and I doubt that's related.)
- Dan