Comment 18 for bug 599017

There is also a possibility that it is a bug in the flashplayer (which I don't have a lot of experience). Some googling suggests there is a configuration parameter which sometimes works around GPU issues that might be worth trying if you haven't already:

sudo mkdir /etc/adobe
echo "OverrideGPUValidation=true" >~/mms.cfg
sudo mv ~/mms.cfg /etc/adobe/
# http://blogs.adobe.com/penguinswf/2008/08/secrets_of_the_mmscfg_file_1.html

Apparently flash has some buggy code for determining whether it can use the GPU for acceleration or not, and this lets you bypass those checks. In theory doing this gives better flash video performance. Possibly this parameter could let you control the rendering path in general, which might let you bypass the bugged code that freezes.

If there is already a /etc/adobe/mms.cfg file present on the system with this setting, you could try setting it to false.