The low idle clock speeds are normal.
It is simply the chip hitting it's power saving state.
You are probably lagging in games because you have overclocked to far and are overheating under load.
and use it to handle your overclocking and keeping track of your temps.
After you have used Rivatuner to properly overclock your card, use ATI Tool
to test your overclocks.
Lower your overclock or use Rivatuner to raise your fan speeds as needed.