PRAM batteries can last a LONG time, but 3 years is about normal. Try changing other settings, like mouse cursor speed. Does it keep the proper date and time? If it can not keep date and time or mouse speed, then the PRAM battery is bad. If the PRAM battery is a known good one (new) then the capacitors on the motherboard are probably going.
If those settings above are keeping and memory settings keep switching, you can try a PRAM reset or deleting the memory preference file.
If those settings above are keeping and memory settings keep switching, you can try a PRAM reset or deleting the memory preference file.