Create post, blank page on post.php results

I’m having trouble with a WordPress 2.5.1 blog (hosted on a shared Pair Networks server).

If I write a draft post of a line or two and save it, no problem. But when I try a post that’s more than a certain number of characters — I can figure out how many if it helps troubleshooting here, but figure a 4-5+ sentences — the post doesn’t complete and the PHP page (post.php) comes back blank.

This just started happening on a server where there have been no changes. It was running 2.5; upgrading to 2.5.1 didn’t help. There are 120 good posts in the blog already.

Thanks for your help.