I’m not sure when it started, but lately Flash (v11.6.602.171) has been stuttering like crazy when playing YouTube, BBC iPlayer or in fact any full-screen video. I tried uninstalling Flash, reinstalling, disabling plug-ins in Chrome and re-enabling, but nothing helped. After googling around for a couple of hours, plenty of other people are seeing the same problem but nobody has found a fix.
In the past, when Flash has misbehaved on my Macs, I’ve installed the beta and that’s usually solved the problem. And sure enough, installing Flash v11.7 beta on 10.8.2 (Lion) solved the problem. I’ve been using it for a couple of days, and I’ve not seen any beta issues. Your milage may vary.
The beta download is on this page.