cursor shakes / Text slides all the way down...

[FONT= "Garamond"][MS WinXP Home 2002 - SP2; OE 6; IE 6; MS Office Pro; Dell Dimension XPS T448MHz Pentium III; 384MB RAM; high speed cable.][/FONT]

Hello, if anyone experienced the following, would you kindly let me know why and how to fix?

It happened twice in the last 2 weeks. I'm typing in my Outlook Express and suddenly the cursor starts shaking and flying and the entire text rapidly slides all the way down! Nothing I did would stop it from running down, let alone bringing it up again. Even trying to start a new message in another window it would do the same again. I wanted to run the Malwarebytes' antiMalware but it wouldn't open. I lost the text of course...

I left the computer alone for an hour or so and when I returned it was ok so I hastened to run that malware thing thinking it would catch dozens of infections...but it didn't find anything! Very strange I'd say!

Thanks ever so much for any suggestions. Adela