Video playback stops working after a while - Ubuntu Studio Precise

Hi, I've had a bit of a search for this topic but haven't been able to find a solution. Sorry if it has been covered elsewhere!
After editing for a while on Kdenlive the video playback stops working and just gives me a grey preview screen, although the audio still works. This happens in both the Project Monitor and the Clip Monitor. The only way to fix it is to either reboot or switch to OpenGL for video display. I don't like doing this however because the playback is choppy and I get weird artifacts around the preview window.

I have an AMD Athlon quad core and an ATI Radeon HD 4250 onboard graphics card
Running Ubuntu Studio 12.04 (64 bit I believe)

Any idea? Is there a way I can maybe reset my video card?

Have you tried using the proxys feature and see if you get problems with playback.

What format is your video source? HDV mpeg2 ?

You could try to reset X

Hold down those keys: AltGr + Print Screen + k

(on some keyboards AltGr doesn't exist, then try the key right to the space bar)