I would say you may have temperature problems, sometimes when computers overheat they kind of down clock (temporarily) to lower temperatures. Download Hwmonitor and play a game and printscreen the temperature ranges shown.
Yeah, first check to make sure it isn't overheating. Download CPUthermometer.exe (forgot where I found it but it works pretty well to a point). Check to make sure it's below 30 C when idling (doing nothing). If it's okay then check your power supply rating. You should probably know since you built it. If it's less than 500 watts, you probably can't run the GTX 580 very efficiently. It's a power hungry card. Also, power supplies can break down over time and deliver less power, so that could rob your system of adequate power. This might be the case since you said it's getting slower and slower over time. If that all checks out fine, then run a virus scan. I know I sound like a tech support guy, but it might work for you.