losing ability to edit posts – pages seem okay

This has gradually become worse in the last couple of months:

I press the publish button and it seems to take an age and ends up with a white screen. I go back to ‘manage’ page and the post is there in the list – all looks well – but if I click on the post (i.e. to edit) I get the normal header; black top line, blue line and third line of: posts, pages, links, categories etc., navigation line, below is white space – not even a footer.

I would say this has gone from occasional to maybe 40-50% of my posts (same happens for contributors) but so-far never with pages. If a post needs editing, all you can do is copy from the blog and make a new post deleting the old.

I monthly use my WP-DBManager Plugin to ‘optimise DB’ but this seems to make no difference.

Is this a known 2.6 issue and will it improve with 2.7? I don’t remember it with 2.2-2.5…
Thanks