Memory allocation with WPMU DEV SEO

WP 3.1
Original Memory 190M

When enabled I noticed on some sites when viewing image pages or front pages that I got a memory issue:

Fatal error: Allowed memory size of 199229440 bytes exhausted (tried to allocate 523800 bytes) in /wp-includes/cache.php on line 93.

I upped the limit to cover it, so took it to 500M, I realised I installed WPMUDEV SEO, Market Press and a good few of your other plugins so though maybe I might have pushed it high but this seemed way to high, anyway I made the changes to see. I cleared the error above but now I get a 500 Internal Server error with the:

'Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.'

Disabling the SEO plugin resolves the problem.

The problem first became apparent with the latest version of Podpress but it does happen on one site running only one plugin (mappress) but not on pages where the plugin was called.