I'm not the expert here but Compiz could be to blame.
When I first installed Ubuntu 10.04 with Ultimate Ed I had to change my settings in Compiz several times to stop drop down menu's from crashing. And to stop graphics crashes-
I don't think it was my graphic's card.
The Ubuntu forum is a good place for info. about your system:
Not sure if this is your case but I'm trying to help-