Single Page Permalink Problem

Hi, I have got this very strange problem whereby a single page (in this case a custom post type page) permalink just stops working sending me to the 404 page – after a while it comes back but then promptly stops working again!

I have tried flusing the rewrite rules – if I change the permalink to something else then the page works fine again but when I put it back to the original it stops working again! All other pages (both custom type and normal) permalinks all work fine – its driving me nuts

has anyone encountered this before?