Unknown Computer Problem

Hey, my computer keeps acting strangely. What it does is it will skip every once in a while (audio and visuals), and freeze at times. While gaming it will sometimes freeze, or the screen will artifact heavily at times. Other times, my computer acts normally. My CPU is fairly new (got it this year), the RAM is new (got it around February), the GPU is new (got it around June), yet the motherboard is pretty old (around 5 years old now). The HDD is the newest of them all, as I replaced my old one, thinking it was causing this problem. The PSU is also 5 years old, and I don't even know its wattage.

Would anyone know what the problem is? I'm thinking it's the motherboard or the PSU. I'll inevitably replace both, but I was wondering which would have the most likely cause? I also know this isn't software related, I have all the latest drivers and no sort of malicious software on the computer.

CPU: E8400 3.0 GHz Intel Core 2 Duo

GPU: MSi TwinFrozR 260 GTX:http://www.newegg.com/Product/Product.aspx?Item=N82E16814127464&Tpk=twin%20frozr%20gtx%20260

have you disabled any unnecessary services running? What games is it happening in? And have you checked for viruses?

I may be out of it but maybe your PSU is underpowering the new video card. I have a twin forzr 275 and know it uses alot of power. Just a thought. Artifacts are usually attributed to video cards.