Check for swelling capacitors. My anecdotal experience is that in 2-4 years, if a motherboard has some low-quality liquid filled capacitors, you might see some swelling in the top of them. The behavior of the computer is very similar to memory problems; lock ups, blue screens, but also include other oddities like a USB device suddenly not working until you plug it into a different port, video stutters/stammers, or even getting a memory-gulping app crash.