Same thing happens to me, my workaround is to close the designer tab as soon as I finish making any changes. Have you checked MSDN Knowledge Base for any articles about the problem?
EDIT:
This is the closest article I've found:
http://support.microsoft.com/default.aspx?scid=kb;en-us;825007
It deals with controls disappearing, but it doesn't say anything about properties being reset.
If you try the workaround in the article, please let me know if it works.