All-day Outlook events sometimes don't show up

I have Outlook 2003 with SP2 on an HP laptop with XP Pro SP2 build
2600. The laptop case is closed, and I use as monitor an HP 2035 LCD,
which I use in portrait mode using PivotPro. My problem is with
all-day events in Outlook. Some all-day events do not show up in the
5-day work week view when I use the 1200X1600 resolution.

- If I change from all-day to specific hours, they show up
- If I go to 1-day, or 7-day, or month views, they show up
- If I change to a resolution of 768X1024 or 1024X1280, they show up
- If I double click on the gray area on top of the calendar, it brings
up the event to be edited. So, it's there, it just doesn't show.
- If I switch the monitor to landscape mode, the event shows up in all
resolutions, including 1600X1200

So, the only the combination that doesn't work happens to be the only
one I like to use! What am I doing wrong?

Thanks,
Reza