I think having "smooth scrolling" on can cause this. You can turn it off in
System Preferences -> Appearance.
2 - On a muli-page file the PgUp and PgDn keys have
instead of scrolling up or down a page at a time, the window jumps
over the place. For example, on a 5-page textfile and starting with the
window about 4 pages down, hitting PgUp puts the window at about page 2,
hitting PgUp again puts it at page 3, hitting a third time puts it at
the start of the file. This makes no sense to me at all. OTOH,
clicking on the scroll bar moves the window one page up or down as you
would expect. What might be causing this behavior?