You may have bad memory stick. How long have you had the system?
Oh man, i've had it for at least 5 years.
How can I verify this?
Verify that your system fans are working. They circulate air through the laptop reducing heat buildup. If your fans aren't working, they need to be repaired.
Second, I would reseat the memory stick. Take it out of the slot, blow any dust off the stick and the slot and reinstall it. If things improve, you're good for a while longer.
If the freezing behavior reappears, best thing would be to just replace the memory stick. Due to heating/cooling, the memory stick separates from the slot resulting in the freezing behavior.