What kernel and what grafic card do you use?
If you have default Ubuntu 8.04 kernel,upgrade to new linux image which is now available in the repos,maybe it will help.There are lots of similar bugs reported but they all depend on the hardware and the kernel.
Check launchpad.net starting from here
Post errors that you see when starting apps from the terminal,if any.
Also check your kern.log
for any errors and post them here.