Some famous scientologist actor once said, "Help me help you! HELP ME HELP YOU!"
We need a little more info bro. Like...
What OS are you using? It would help so we can tell you what tools to use to figure this out.
Do you have any temperature monitoring utils installed? In windows, I can monitor my 7600GT temp. Maybe your video card fan stopped. Is it overheating? My fan puked a while back and I didn't know it. My card played fine for the next month but gradually decreased frame rates and then hung my system altogether.
What map were you playing? Does it seem like it only occurs on certain maps?
More importantly, if you play on a server with a lot of peeps vs. playing on low player servers in the past (when you had no probs), you will suddenly notice an FPS drop in certain map spots - especially long open areas.
You get the picture. We the doctors, you the patient. We got the latex on, so cough it up for us.