Problems since overclocking 850pro - help please

Hi again. Second time I've needed advice from the board. Hopefully you can help out. Will try give as much info as possible. If you need anything further to pin point it please let me know. NEED THIS SORTING!!! :x

Ok bought new 850 pro. Flashed with xtPE bios. Then had 16 pipes and clock running at default xtPE speeds of 540/590. Bought and installed after market cooling - Zalman vga cooler VF700-CU LED and ram heatsinks. Happy.... not for long.

Playing BF2 and system locks after 1 hr. Rebooted, no errors. Tried again same thing. googled the problem. Found o/c'ing uses more power. Bought new, expensive :x 500 W power supply as the old one was only 230W (so it needed doing anyway I guess). Won't run at XTPE speeds. When it crashes the screen locks, still have sound though then the mouse locks and I need to reboot.

Card runs at 38degrees when idle. Tried overclocking tool in ATI Cat ver 6.4 and discovered the same crash. Clicked the auto button and while it was slowly notching up the clock speeds I got the same crash. Temp displayed maxed at 53. Cooled a little into the 40's. it then raised some core speeds as it does and the screen locked, mp3's playing in the background. Waited but nothing happened. When MP3 track finished and nothing. Needed a reboot.

When clock speeds are back at 500/500 BF2 runs perfectly. Had a 6 hour ses with no problems.

Any help at all would be awesome. Want this fixed and running at best clock speed possible.

Gaz