With 1.5 GB of RAM, it's highly unlikely that your system's virtual memory is even activating, unless there are a lot of programs running when you play games.
But see if increasing the VM's size keeps things running smoothly.
If not and the problem resurfaces, then reseat the video card making sure it's fully seated. Sometimes they need a healthy push to properly seat.
If that does not help, then make sure the video card's fan is running, the fan is properly and tightly mounted on the heatsink, and clean out any dust built up in the heatsink.
Also, clean out any dust in the CPU's heatsink.
If those are ok, then remove the computer's cover, and run an external fan.
If that works, then your system needs additional cooling fans.