Inserting Graphics During Post

[Onomatopoet]

Hello all,
I love learning the hard way, so I hate to ask for advice, but this is an issue I've spent weeks researching but just can't seem to work up a fix for. To sum up the issue, I've got an old computer that won't POST with any type of graphics card.

Never used the x16 slot before, so I was worried it might be bad. All system drives spun up and fans kicked into high gear as per usual, but then they just kept spinning at top speed instead of moving onto POST and booting up. Everything except the mobo did its self tests. No BIOS, no POST, no beeps, nada. On the other hand, all systems function perfectly without a card inserted.

Fixes I've tried:
Cleared CMOS at several times
Tested three PSUs
Swapped graphics cards three times
Updated BIOS
Rolled back BIOS
Updated BIOS
Uninstalled all old drivers
Reinstalled all new drivers
Disabled devices, reenabled, etc.
Updated chipset drivers
Tinkered with all sorts of crazy BIOS settings that do and don't affect graphics
Disabled onboard audio/video
Set primary video to PCIE
Checked and rechecked all cables(yes, even the card's 6-pin)

Strangely enough, if I power on without the card inserted, pause for about three seconds while fans run full speed, then insert the card just before the fans slow down and POST finishes, then everything works like a frakkin dream. This means my x16 slot is at least partially functional, but implies there's some nebulous switch somewhere I'm not putting at the right setting. Like a jumper that doesn't exist that I need to engage.

Current system specs:
-Corsair TX 750w
Single 12v rail with 62A
-Pentium 4
D915GRO mATX mobo
i915p chipset
82801fb southbridge
only one x16 slot
-3GB RAM
-Windows XP Home
-XFX HD 5770

It's an older system, like 2006 or so, so might an issue like this be from IRQ conflicts? I know during the bootstrapping somewhere it magically coordinates and allocates that stuff, but msinfo32 says I've got USB running with my graphics. I'm really at a loss at this point. I can't be removing the card every time I need to power on, not to mention how dangerous it is to insert it while the machine's powering up. Any ideas about what happens in those early stages of POST, and why I have to bypass them to avoid my hangup? Why would the system fail POST and not tell me?

 

[codypilot]

A similar thing happened to my moms pc. First thing's first, are the drivers for the card downloaded and installed?

 

[Onomatopoet]

Oh yeah. That was one of the first things I did.

The card runs perfectly with my system, barring the failure at POST.

 

[Onomatopoet]

The old lady wanted me to get rid of it, but I figured I could convert it into an HTPC instead, maybe stick in a low profile graphics card for HDMI connectivitiy. This would have the added benefit of older game playability, since the onboard graphics won't play a couple of my kids' games, but unless I can get the x16 slot working, I'm hosed.