Random lockups

I had Vista x64 installed with 175.16 Nvidia drivers and it went fine for several month until this month when it starts to do random lockups. It happens in just surfing around listen to music or playing games. It just stops instantaneously and I need to do a manual reboot. Heat on all hardware is checked, everything is ok. I installed new drivers, 175.19 and still same problem and later I could not almost enter the game because it instantaneously locks up and one time a blue screen.

I formated and install Win XP x86 with 175.19 drivers and I just played a Wc3 game and it locked up again...

My GFX is 8800 Ultra. I am not sure if it is graphical related but this is my only suspect atm because in the blue screen I had 3 sec to read and I got something like "graphical drivers". When I installed new drivers when I had Vista it slightly changed from just going black a couple of seconds when playing a game with a lock up to follow to just lock up right away.

The only change I have done so far to the point when it started to lock up is adding a amplifier with floorspeaker which is connected with an optical cord. Before I just had a 2.1 PC speaker connected through USB. Also I have changed the water in my liquid cooling tank. Nothing else really...

Any ideas? really frustrating.

hmmm not good , suggests dodgy ram module , which if you havent overclocked your system at all then it needs replacing , make sure your rams running at its recommended voltage and speed rating first

run memtest to check just in case.

I will do a memtest later today, I will first play for a while and see how it goes. If it still remains error I will probably buy new RAM instead of RMA, I actually had this in mind before the errors occurred.

I will let you know how it goes. Thank you for the answers.