I need expert help on this on.

I have a Radeon 1950 Series graphics card and i have had it since the start of decmember. Every time i play counterstrike for over an hour it starts to jerk alot then it freezes still and i cant do anything the only thing i can do is restart. The sound jams in the same place and just stays.

Is this because its overheating and how do i stop it from overheating or prevent it.