ok, temperature is not the problem but a little high, being a intel with more than one core it is in a decent range if never goes over 70°C
still you haven't mentioned your video card or driver used, it is important with flash, also if you are using kde 3 or kde4(includes the use of compiz and kde4 desktop effects)
the reference number doesn't say anything, even on the acer website but in newegg it says integrated so it could be a intel gma4500 or a 3100, not sure which one is the right driver for it
in synaptic, can you reinstall the flash plugin?
also when watching a video, pause it and on right click on the video/settings
in the first tab called display uncheck the enable hardware acceleration, if the settings option is grayed out, open a video in youtube, it shows that option
after this, close firefox(not reload, close the app completely, all instances) or any other web browser, then open again and see if has any effect
this will make the video run entirely on cpu and won't try to use video card, there are other tricks for flash that can be tried