Full Screen Flash Video Choppy - Amazon, Youtube, etc.

I've read through countless discussions regarding choppy video (when full screen) when using Flash. I've got a couple of Minis (2011) that have no problem playing 1080p MKV files via Plex, but having a struggle to even play 480p content in a browser at full screen. When windowed they play 1080p flash content smoothly, even at double size. As soon as you go to full screen, it becomes unwatchable...about 5 frames per second. Safari, Firefox and Chrome have mixed results. Chrome does the best job and will play smooth 1080p full screen flash content sometimes and then other times becomes choppy. I can then hop into Plex and play any other 1080p content flawless. I've tried deleting the Flash folders in ~/Library and also deleting cache/preferences and updating to the latest builds. It's really a strange phoenomena and can't seem to put my finger on it. Both Macs are running Mavericks, but I've had this problem in previous versions of OS X. I've even reinstalled from scratch before and this problem always seems to come back.

It don't see how it can be a CPU/GPU/RAM issue when I can switch over to Plex and watch high quality 1080p content full screen and then not be able to watch a 480p flash video in full screen. Help!