GTO2 Poor Clock Speeds :(

[kahlos]

Hi
Bought a GTO2 a few months ago (in Nov) and i've just put it in a system and tried overclocking it with ATI Tool. I only managed to get 450mhz Core and 525mhz mem. I've put a new ATI Silencer 5 cooler on the card and my case has too much air flow. I used the preapplied thermal paste which was on the ATI Silencer including the pads for the mem chips. I've also flashed the bios to unlock the extra 4 pipelines. The previous owner managed to get 500mhz core and 600mhz mem with the stock cooler !
This is really annoying! Is it something to do with the preapplied thermal paste or summin ?
I start getting artifacts at 55ºC on the core and 50ºC on the memory. Is this normal ? Is something fried on it or summin ?
I got the readings for the temps on ATI Tool. I made ATI Tool put fan speeds to max. So whats up with it ?
The previeous owner had it for 4 months. So should i try to flog it off and get a new one or is it summin to do with the cooler and stuff?