You should try for the clock on the EVGA GTX 260 SC, here's a screen of mine:
GPU2 is running a F@H client right now, hence the higher temp.
That's your's in the background. I wonder if the have the same memory chips, etc.
If it's slowing down it's probably because of the extra heat being generated. The cards will throttle back to prevent damage, artifacts and craching. Of cousre your using Afterburner so you would see if they are hich. Probably need more voltage too to sustain the higher clocks. Mines at 1.1800v using GPU-Z to read it.