^ Concur, normally memory problems
With a 3 year old system, I would first just Remove and reseat the memory modules (bout 2 Xs). Tewo problems with them (a) over time the contacts may develop a small amount of oxide and resesating a couple of times removes this oxite. (b) Due to (1) thermal expansion/contraction and small vibrations one or two contats may no longer be making good contat.
Also download the memtest86 ISO, burn a This ISO to a CD (Makes a bootable CD) and boot to the CD, run memtest for 4 -> 8 Hours to test your memory. Caution Note, don't just copy the iso to a CD, you must use the "ISO burning function.