sorry ii got it wrong,
its 300w

O.K. now that we got that straight .
I recall you saying that you get the problem at the "same "place ( MSnger 7.5), is that still true? If yes
I believe your problem are software related. Hardware errors are often random.
Try shutting off processes and programs one at a time with some time in between to see if you can find the culprit.
Also take a look in Event log any red stop errors are very important.
Let us know what you find.

Kirock, if you don't know wtf you are talking about don't just give advice anyway, you could end up doing more harm than good.

So, did you finally sort the problem out?

You should keep a Ubuntu Linux boot CD/DVD so that you can run the PC using that OS. If it works without causing the symptoms, you know that it's a software problem. If the problems persist, it's a hardware problem.

I still think it may be the power supply. I know you corrected yourself and said that you do have 300w, but it could still be a bad power supply. try replacing it. if the problem persists, return it and try something else!

Find a different video card. Immediately. Test it with something else, anything else. I've heard more horror stories than you'd care to know from Radeon 9700/9800 series cards, and my own Radeon 9800PRO that I owned back in the day gave me almost this exact same problem.

Now I'm not sure if the video card is the problem, but I've certainly seen that before, so my first piece of advice would be to try another video card, preferably one not from the Radeon 9000 series.

Also, the power supply has a very low +12v rail, if I read your post about the voltages correctly. 10A is absolutely dismal for a +12v rail, so it would be in your best interest, stability issues or no, to upgrade your power supply to a GOOD BRAND. GOOD BRAND means buy your power supply from a company known for quality. Enermax is my personal favorite, but PC Power and Cooling, OCZ, and Antec are some other good brands. I'm sure lots of people on this forum could think of more good brands too.

is it a laptop or desktop
either way
maybe your heatsink is not connected properly
that helped me tremendously