[1] PC or Mac? What version OS?
[2] When you say "leave Quark on for a long time", do you mean actively using XPress for a long time produces the virtual memory problem, or can this be reproduced just by leaving XPress running for a day or so with no such activity?
[3] Do you use any third party XTensions?
[4] You specifically mention version 8.02. Does that mean that you think this might be a problem introduced with that version?