Wow that is wierd, i have one 775 board(p5p800-vm) that has a i845g + ich5 chipset and i have it running perfectly, i updated the bios to the latest version(otherwise i had issues with the bios complaining about the uCode of the cpu).
Which bios version are you running?

How much ram does the PC and Video card have?
I vaguely remember similar issues on a 865 chipset and it was down to the 2 or 4GB Ram, 256MB graphics or maybe even because I forgot to turn off HT.

Basically it wasn't anything to do with ACPI at all

512MB SD-RAM, and a Riva TNT2 M64 32MB card (ASUS brand). Apparently, there was no way to adjust the AGP Aperture Size, which normally on an OEM motherboard is 256MB locked (no option in the advanced BIOS setup either), which was the issue. My Socket 370 system would complain as well with the GeForce 6200 AGP card installed until I switched the Aperture Size to 64MB.

I gave up and put the Socket 370 board back in, got almost every connection in right the first time, except for the HDD and Power LED connectors (I put them in backwards), installed the GeForce 6200 AGP card and VooDoo2 card (some games don't work with the GeForce 6200 card at all, for example, Jimmy Neutron Boy Genius), and all is well. Had small issues when I was installing the VooDoo2 card (a window popped up saying the card wasn't detected, so I moved the card, and it now works), but everything's running smoothly. Had to re-install certain programs to get some games to run on my machine, and I'm going to re-install the PowerDVD program.

I ended up getting the rlowe memory patch which seemed to fix all my problems, if you still wanted to mess round with the P4?
I would use the demo first just in case it doesn't work for your situation.