Mouse hover over submenu item does not expand menu until I leave and re-enter item
Solution 1:
This could depend on your chosen theme. I've read a while ago, that someone had the same problems with the clearlooks-engine. He had chosen another theme and the problems were gone.
You can try to disable "Mouse Position Polling" in compiz.
Install CompizConfig Settings Manager:
sudo aptitude install compizconfig-settings-manager
Then goto System/Preferences an chose "Advanced Desktop Effects Settings" (i'm not sure if it is still called this way, because i use ubuntu in german) and disable Mouse Position Polling:
