Hi Will
If you have not already tried – increase the memory limit in wp-config.php.
I just added
ini_set("memory_limit","150M");
under the line
define('NONCE_SALT', …………………..
and everything just started to work.
Some of the symptoms you describe are similar to my problems. Missing bits of display and, in my case, I had to deactivate a plugin to get another page added to my menu.
Best of luck
Tony