I looked at some of your other post. Turn off compiz and see if that improves it. It can depend on the ram the video card has. Need to be at a little over 16mb for good video playback from what i have been told. Like 18mb video ram.
Distribution: RHEL 5.1 with Pieces of this and that.
Kernel 184.108.40.206, KDE 3.5.8 and KDE 4.0 beta, Plus Development src.rpm, ATI fglrx64_7_1_0-8.433-1 rpm with 3D and DRI working.
Acer 5100-5840 with webcam, ati, sdcard reader, sound, atheros based wireless, all working. Only thing not working is the memory stick reader.