Odd. And annoying.
I'd be inclined to nip down the scrappy and get another cam cover. They're two-a penny round here anyway and will only cost a few £.
The old one may be mis-shapen in some way, hence the dissappearing gsket when it gets hotter. Clean the 'new' bit up really well, as well as the mating surface on the cam carrier. Then use your favourite gasket sealer and another new cork gasket, and keep your fingers crossed it seals this time.
Probably is the cam cover
mine cam cover gasket had been changed genuine vauxhall on MAY 2003
2 months later on July 2003 i noticed new oil around the cam gasket
i phoned to a friend of mine who is a vauxhall new parts dealer
and told me that every two months they send the gaskets back to Vauxhall
3 weeks ago i put a new cam cover gasket and a new oil cap and if there is a link again they told me that is the cam cover for sure