[ubuntu_mate] ATI Graphics Card Drivers

I received an old laptop, an ASUS A6Q00VA, with intel pentium M 750, 1Gb of RAM, and an ATI X700 Mobility Radeon Graphics Card, and installed ubuntu mate on it. It runs really well, but when i try to play youtube videos, the sound is fine, but the image is really bad, stuttering frequently even at low resolutions. I don't think these problems existed when it run XP. Maybe it's a drivers problem?

Some more info:
When i run dmesg, it shows no boot errors for the card
The output of lspci | grep VGA shows

03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV410/M26 [Mobility Radeon X700]