Sounds like a RAM issue though you seem to have loads, could be a Video card issue or maybe an issue with background services or frontend system demands.
Most likely however I think your PC may be getting too hot. Video cards and ram get laggy if they get too hot (happens on some laptops where cooling is insufficient. It's likely the launching/exiting of games is demanding quick changes in disc access points too, generating heat as your hard drive spins faster. Try a full, alphanumeric sorted defragmentation using O+O defrag to see if that helps, and try running the PC outside the cab for a while too.