Have you tried reseating the memory and CPU?
It could also be the battery that is causing the checksum errors according to one site that I have been looking at
http://www.pcguide.com/ts/x/comp/mbs..._Corrupted.htm
I would have a read of this and also the pages linked, especially the one on troubleshooting the motherboard. Another site worth a glance is
http://www.computerhope.com/beep.htm