Anyone experiencing really bad overclocks with the Visiontek 4850? Is it because they use a lower stock voltage?

It seems that my card starts artifacting bad in ATITool at around 670/1050 with temps of mid 60c. I tried two cards so far and both yielded the same results (both VT cards).

Is it the stock cooler or just the voltage settings on the card? (I tried my latest OC test at 65% fan speed).