Graphics problem after suspend / resume

I've just installed Trisquel 4.0.1 64 bit version on my HP Pavilion dv6 laptop, and am having some problems with the graphics after the laptop suspends and resumes - the screen flickers and jumps quite badly. I haven't seen this problem from a clean boot, and hibernate, resume generally fixes it. I can't see any problem in dmesg, but I'm not real sure what I'm looking for. The laptop has a Radeon HD5650 graphics card.
Can anyone help me track down what's going wrong with my system?