Hardware monitor reads 0.6V in the 12V ?

My computer has an ASUS A7S333 and the Hardware Monitor of the motherboard started to show PROBLEM DETECTED. Once i come to the BIOS setup to see whats going on , the 12V reader show 9V. I kept checking it since it was working fine. In some days it came back to 6V and i took it to the assistance. They gave it back to me saying no problem was found. I took it home and the reading was even lower : 3V. In the assistance again they kept it for 2 days and said IT WAS OK ! At home , with this only reading as the problem, i started to have system instability and hangs. I tryied to reinstal my WinXP Pro but i hang every time. Than i came back to Win 98 release 2, a lot more light and i could finish the install but started to shutoff suddenly without notice. It was ok running whatever i was running and shutdown completely like if you shut the power of the house off. I took it back once more to the assitance. After 8 days they said they changed 1 256 Mb DDR333, the CPU ( an AthlonXP 2000 ) and the cooler ( ???). Back at home i was relieved. No more hangs and shutdowns but the reading of the 12V was still showing 3V and hardware monitor kept showing the problem. I decided to keep the equipment while it was running ok. Today, i started to hang and shut again and went back to the Bios setup . The 12V reader is showing readings from 0V to 0,6V.
Some one could tell me, Please, What The.. is going on in my PC ? I thought that after all this only here i could hear a wise answer.
Thanks for you time