I have found that if Extensis QX-Tools 4 is installed in QuarkExpress 5, and the Document QX-Viewer is "shown", that there is a massive and ongoing memory leak, about 1MB every 5 seconds. To solve: First open Quark, then your document, then go to extensis and "hide" QX-Viewer. Close Quark. Then reopen it. The memory leak should not occur now.
All the other QX-Tools seem be ok, but you might consider removing the Extensis QX-Tools plugin extensions.