Fire up the Task Manager and look at the Performance tab...keep it open. Play your games, in windowed mode if possible or on another monitor. When the lag happens check the Performance screen to see if RAM is being maxed out or if the CPU is at 100% etc. Also keep HWMonitor open and see what the temps are at the same time...'I think so' doesn't really help us diagnose your problem. Help us help you.
Also check that your anti-virus isn't kicking in at the same time, might just be running a scan. It would be an idea to run a defrag, CCleaner sweep and a virus/malware etc scan too.