html5 videos and flash videos drop frames

When I watch videos picture freeze on few seconds, but sound still playing. When I watch videos and use player interface or use mouse - videos lag worst, but sound still clean. When I click right mouse button on flash video - picture freeze and still freeze while right click menu is active, sound playing 10 seconds and freeze too (html5 video is normal when click). I tried off hardware acceleration of firefox and flash, safe mode, old versions of flash and firefox, different videocards like a radeon r9 270x and geforce gtx 550 ti. I saw that problem on the computer of my friend. Now I use Chrome for videos, because the problem is very annoying. I read about the problem is actually for Windows 8.1.