Hello,
I have just taken my Zaurus 5500 apart and put it back together and have run into the same problem. Since the screen was removed and everything I can only think of two possible reasons for this happening.
The more optimistic reason would be this occurs on all the Zaurus 5500's, but I have somehow placed my screen to high. This seems logical since the dashes don't seem to occupy any space I need for viewing the screen, but I could be wrong since I'm judging this by looking at the cursor from a failed OZ flash.
The second reason could be I have damaged or pulled out the ribbon cable that attaches to the right side of the screen. This could be my problem too since I was having some problems putting it back together. Now, since the dashes appear only occasionally on yours this sounds like it is the case. I hope not, but it is a possibility.
If the problem really bugs me I might take it back apart and see if I could fix it somehow. If I do this I'll tell you how it went.