I know that this thread has been dead for a while, but in the interest of public knowledge I'd like to post the solution. This applies to the Acer 5750G and specifically to the PageUp key bug.
The problem is not X or Linux(it shows the same behavior on Windows, only there the bug is impossible to notice because of how Windows treats key events), it's the BIOS of the laptop. It doesn't generate the correct keycode on release of the pageUp key(instead of 33 or whatever it is it generates 255, which is unknown key). That's why the laptop behaves like you kept PageUp pressed and scrolls to infinity.
Solution? Go here: http://support.acer.com/product/defa...x?modelId=3475
and update your BIOS version to at least 1.20, and the bug will go away. You will need DOS or Windows for that.
A word of advice: updating the BIOS carries the slight chance of bricking the laptop. Please inform yourself and know what you're doing. You're doing it on your *own* responsibility.