lga775 System Showing Strange Issues with POST, Can't Pinpoint the Problem

[rtey31]

I'm building a budget system for some gaming. The specs I'm going to have are:
- E5450 (with lga771 to lga775 mod)
- Asus P5ND2-SLI
- 8gb DDR2 667mhz
- HD 7870
- 80+ rated 500W PSU

I just got the motherboard from Ebay (marked as used and confirmed working), and all other parts are confirmed working. The motherboard came with a Pentium 4 and 3gb of DDR2. Tried booting with the HD 7870, get fans but no display. Tried swapping in a C2D E2160 and my RAM, no display. After trying a bunch of configurations, I got somewhere:

- Pentium 4
- Asus P5ND2-SLI
- 8gb DDR2 800mhz (different kit)
- R9 270

This configuration boots up sometimes. When it boots, the fans spin slowly, the speaker makes a short beep, and the BIOS splash screen shows up. When it doesn't boot, the fans go at 100%, the speaker makes a long beep, and no display. I find that when I try to boot again, it randomly sometimes boots and sometimes doesn't. I tried lots of things, including the no display signal guide. Here's what happens when doing a few things, I think it might help:

When trying to flash BIOS:
I do this with a bootable USB as this motherboard apparently doesn't use EZ Flash 2. I copied the AWDFLASH.EXE and the actual BIOS to a USB, but when trying to boot from it, the motherboard doesn't recognize an operating system.

When trying to boot into a Windows HDD:
The Windows Recovery screen appears for a few seconds, then the system reboots, and same thing repeats.

When trying to boot into a Windows bootable USB:
The Windows logo appears and stays there, never doing anything.

It's also worth a mention is that whenever I get a proper boot, I get a message saying CMOS checksum error, and that the BIOS had been reset.

I'm at my wit's end here, I don't know what else to try

 

[rocktalkrock]

Given this motherboard is about 12 years old you might try replacing the CMOS battery, it could be weak or almost dead. If the battery is very weak that could explain why it sometimes boots and sometimes doesn't, and why you would get a message saying "CMOS checksum error, the BIOS had been reset".