Tiny MCE

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 2010 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.

You might also want to look through the 3.5 master list – https://www.ads-software.com/support/topic/troubleshooting-wordpress-35-master-list?replies=4