Lag in games after 45-60 mins. Win7 64

[Jimmy420]

Problem Solved!

I'm having an issue when I play games for over 45 mins or so that my PC will start to slow down and lag a lot, making the game unplayable.
My first thought was overheating but I've been running CPUID HWMonitor for about 4 days now and my temps are fine, even when playing. I checked temps on my GPU, CPU, HDD & mobo just to be safe and nothing is out of the ordinary.
I also ran Prime95 for 10 hours to check for memory errors and everything is fine there as well.
Lastly I've used a combination of HiJackThis, CCleaner and Spybot S&D to remove any possible malware I could find and I'm still having this problem.

EDIT: One thing that I just noticed after posting this that is pretty strange is that my RAM randomly spikes up to 90% used and stays there for a while along with the first core of my processor, even when I'm not doing anything. wmpnetwk.exe seems to be the one hogging all these resources which I believe is associated with my Xbox 360 media sharing, but my Xbox isn't running when this happens. I'm going to try disabling this and seeing if it has any effect.

EDIT2: That's what it was. Problem solved!

Does anyone have any idea what could be causing this?