I've seen this problem before.. It has to do with a special type of
acceleration called something like hardwarecursor.  Read the card documentation
for your card, in /usr/X11R6/lib/X11/doc.

Somewhere in there, or in one of those, is a way to shut that option off. 
(supposedly it works on some cards, Ive never seen it work, and it allways
makes a mess.  At least, that sounds to me like what you describe.

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Tim Rightnour    -  root@garbled.net
http://www.zynetwc.com/~garbled/garbled.html