I can't reproduce your problem. I'm using the latest Adobe flash in safari, no html5 trial, no other plugins (except for adblock).
Have you tried:
- using another browser (not chrome though, that one has it's own flash)
- fully deleting and reinstalling flash
Are you using gfxcardstatus or something else to control the integrated/dedicated GPU switching? You could try to force the mac to always use the dedicated GPU (system prefs -> energy saver -> uncheck "automatic graphics switching").
Did you check your flash settings? Do you allow sites to save information on your computer? Some videos don't work properly if you don't.
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By the way, my video playback is fine... no excessive CPU usage, no abnormal heating, and it stays on the integrated GPU for all videos I tried. It performs much better than Chrome flash for some more demanding sites. Initially I bought into the flash bashing - now I think it's mostly an overreaction.
Html5 is a bit more battery friendly, and should be the future for most applications and sites, but flash is ok!