What is your power supply? I've had problems with my 9800gtx+ throttling down to 301mhz, as shown by evga precision, during graphics intensive situations in game. I found that replacing the power supply with one that had adequate amps on the 12v fixed the problem.
does evga precision show that ur video card is getting underclocked when ur doing something graphics intensive?
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.
Download Rivatuner 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 or VCST to test your overclocks.
Lower your overclock or use Rivatuner to raise your fan speeds as needed.