I have a performance problem with my Mobility Radeon HD 54__ graphics card using default open source drivers. I'm not talking about games but non-Full HD videos. Occasionally video freezes for a short while (sound remains unaffected) and then everything is fine for another while. Sorry for my explanation but I couldn't find words to describe that behaviour. I cannot install proprietary drivers, because they seem to crash Ubuntu (13.10).
AMD writes pretty crappy drivers for linux. But if you want the official drivers, and can't get them through ubuntu, go to http://support.amd.com/en-us/download , and there you can specify your video card, then they give you a download for the drivers.