Dell won't complete the BIOS

I have a Dell Dimension 8300, P4, 3 GB RAM, 1.2 TB HDD, Nvidia 6200, BIOS flashed A07. It quad boots Linux, XP, Vista, OS X (x86) using GRUB, and it has just developed a problem of hanging up in the POST or BIOS as it attempts to boot but doesn't complete. The completion bar on the DELL boot screen gets about 7/8's across and hangs there. The 3 GB RAM is balanced 1,3-2,4 with matched 1 GB and 512 chips and hyperthreading is switched on in the CMOS.

It did this once before when I first bought it, and just required clearing the CMOS and rebooting. Then it was fine for six months. Now it is doing it again, but repeatedly. I added a Samsung 750 GB internal Serial ATA drive and did the quad boot setup several months ago. It has booted fine since. I'm fairly sure Quad booting is not a strain on the CPU or motherboard, but I am new to the GRUB language and do use partition hiding and drive swapping to keep all the OS's happy and thinking they are the only one.

If I wait all night, it will boot in the morning, just as if nothing had happened.... BUT... if I leave it on for several hours and then attempt to boot into another OS, it quits again with this same boot problem. I isolated all the drives, 1 Serial ATA, 3 PATA, 1 external , 2 CD, 2 Floppy (1 external 2X) and nothing. I checked the battery, and it's fine, cleaned everything, pulled and replaced contacts on memory, drives motherboard plugs, power, everything, but get the same thing. I suspected heat, but stuck a huge fan in the half open case and that did no good. Took it in the unheated garage overnight (26 F) Still won't boot.

The BC letters of ABCD on the back of the case go to amber. I get an on screen error message "DSKT" but there is no mention of that error code or of only the BC letters going amber on any site I have found. Occasionally the boot stops at other points and displays different error codes.

Anyone have any ideas? If the consenses is that I need a new mo-board, what is a suggested upgrade replacement for that case (this is a proprietary Intel I believe)? One at least clockable. This Intel cannot be overclocked.