The 840AV 40Mhz board is hard on capacitors, and tend to cause their earlier demise.

However, you seem to have gone through the usual suspects (PRAM battery, loose or defective RAM, etc.), except one: possible corruption of the disk. Seeing that you've gone through most of the steps I would have done myself, I would say the next thing is to check your disks, and system folder contents for corruption. You may have a corrupted extension and/or dekstop file.