Well you've tried a new gasket, so that leaves two things, as you've said. Warped cover or damaged/ case. I'm sure you would have thoroughly inspected the case for damage.
Try lapping the cover on a sheet of glass, that should reveal fairly quickly if the problem is the cover.
Alternatively some cylinder base sealant (three bond, yamabond) 'should' solve the problem.
Check the cover for cracks. If there is a crack, it could be very small and difficult to see.
Lord of the Bings, kangaroo farmer