screen flickers after waking from blank screen

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http://askubuntu.com – Every time I lock my screen and then unlock it again I get a really annoying screen flicker that stays until the next reboot. It's not like a graphic glitch it's like the screen is refreshing at a wrong Hz rate. Any ideas for fixes? Specs: Mobility Radeon HD5470 Graphic card with generic out of the box linux driver, Ubuntu 11.10 64-bit with the 3.0.0-15-generic kernel. **I would like it if there's a solution not involving the ati proprietary drivers, because they have been nothing but trouble on my machine in the past. Thanks in advance (HowTos)