Probably a Video Card Issue. Have you gone to Hardware Drivers in System>Administration>Hardware Drivers and see if there is a driver to be loaded for your video card in there.I say this because you are probably just running with the generic driver right now. Freezup is usually always video driver related. You can open Terminal and type in
to get a readout on what type of graphics card you are using and use
<Linux> - Google Search to look around to see how other users got their video sorted by enterning card name with Ubuntu 9,04.
I run Ubuntu 8.04 Long Term Support because of its stability. Ubuntu 9.04 is the latest and greatest from the 6 month release cycle and I don't run it. So I might not be a lot of help there.