youtube judder 9400m, not 9600 MBP

i have googled don't seem find others problem. when youtube video buffering way ahead of play head judders using 9400m in mbp. when buffering finishes play fine. problem goes when switch 9600 graphics. assumed os thing several updates later , still here. advice? thanks.

safari 5.1.2, snow leopard 10.6.8
9400m 256mb system
9600gt 512mb
4gb system ram

actually, case within bbc iplayer desktop (not streaming). (uses flash). can 9400m cannot handle sort of thing more?

 

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