I am using an 8 core (two quad core xeons) computer with 18GB ram. You would would think that would be more than enough power to run email. Yet Evolution will bog down the whole computer as it bogs down the hard drive. It is some odd bug. On a single quad core with 4GB of ram I have no issues. Thus, if you are using a very powerful computer you might also try a less powerful one and see it that helps. So are you fully functional other than rendering? Like can you model and such for lenths of time to make sure the program is steady? I will throw out some ideas: Do you have to allocate memory and is there enough? If the renderer has to write a file does it have permissions and did you create the folder or check that it exists? Are you using Ubuntu 32-bit or 64 bit and is it worth testing in both? Long shot: could it be video driver related. Are you using Ubuntu or Ubuntu Studio? My suggestions are only an attempt to be helpful and not based on any recent experiance trying to run Carrara. Also In System > Administration > Log File Viewer you might find hints possibly.
