difficulty with forms in tabs

difficulty with forms in tabs

If i change away from a tab containing a partially completed form, then come back to it, all the changes are gone. I have set cache to TRUE so it's not re-loading the blank form from the remote, but it must still be reloading the tab content (ie. the blank form) from the cached copy in any case, so i still lose my changes. How can I keep my partly completed form when I switch to a different tab and then come back to it ?