Sounds to me like you have a defective motherboard. Of course it probably would be worth looking to see if there's a BIOS update first, but I wouldn't be at all surprised if you found some bad caps (google it) on the board.
Of course AMD, and Intel really, both initially sold dual core chips as single core. If one of the cores failed testing, or they just needed additional supplies of single core chips, they would simply disable one core and sell it as a single core. And AMD has always been ahead of Intel on this, where their dual and quad core chips were made out of a single blob of silicon, so there would be no outward way of telling if it was a single or dual core CPU. Intel's early attempts at these things were far more crude, as Intel spent a lot of time banking on the idea of raw execution speed, so they had to really try and retool quickly (and they did so at a remarkable speed for a company that size) and took a few shortcuts in the early days.
Check for a BIOS update, if there isn't one, you're running the latest, or that doesn't solve the issue, then you're probably looking at a motherboard replacement. That, or just living with the quirks of the system and using it until those quirks start growing and eventually exceed your tolerance.
Strange. I have an XPC Shuttle PC machine, built up from about a 2006 vintage barebones system desktop with an AMD 64 Athlon Processor 3200 Plus. When I first loaded vista it recognized the full 2GB memory installed (it takes up to 4GB total, DDR2 PC2 desktop version, 667 MZ), and cliamed the processor was a dual core (it is not), and ran as such, while recognizing the full RAM. A few days after we have the difficulty that it only recognizes half the RAM, 4xx MB from each stick. In the meantime too windows installed its "updates", and I searched and found the drivers for the machine chipset, integral graphics, etc. I only installed what the manufacturer recommended, for the actual hardware on the MB. I am not sure if this or the windows update have removed the recognition of the full memory. I even took the piped cooler block off the CPU to ensure that it was not a dual core, as it first said and ran it as so. I verified what it was on the WIKI AMD processor site, according to its catalogue number, and it is only a single core, as stated.
Why would half the memory capacity disappear, and the machine then later recognize the actual literal CPU as a nondual core after running several days otherwise? I can purchase a reasonable update dual core at about $40, but will it revert to recognizing the entire 2 1 GB each memory sticks??? Why did the first few days recognize the CPU as dual core when it isn't, and the full RAM, when it now won't? How can I get it to rerecognize all the installed RAM? I've taken it our physically and reinsalled it twice, even switching one stick for the other, yet the machine and OS only recognize half of each stick. What to do???