Have you tried:
- deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).
- switching to the default theme to rule out any theme-specific problems?
- resetting the plugins folder by FTP or phpMyAdmin? Sometimes, an apparently inactive plugin can still cause problems.
- re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?
- increasing the available memory?