18Amps is 216watts. And the PCI-e slot can provide 75watts through the motherboard. And we know the top 5870s (OC'd 2GB RAM) peak about ~220watts.
But since this happens on BOOT - your 5870 is essentially at idle and only drawing in the range of ~30watts.
After reading your other post it does look like your situation is pointing at the PSU though.
I noticed you mentioned that your 9400GT works OK and that you've had your video 5870 tested.
Have you tested with just 1 stick of RAM installed? (how much voltage are you giving the whole 8GB)
Also you can test without the sound card installed and both optical drives disconnected (just for testing of course).
I saw that you had tested with all the different combination's of RAM.
It's the intermittent nature of the boot problem that has me puzzled.
Still think a minimum parts test could rule out an issue with the optical drives or the sound card affecting boot.
I have tried one stick with all four sticks. On the advice of XFX support I checked the voltage on the RAM and set it from 1.5 to the RAMs recommended 1.65V. It had been working for a day or two at that point but has sense lapsed back into darkness.
I have tested it without the sound card on one mobo, not both. The results told me little. On the other mobo (Foxconn Inferno Katana GTI) If the Sound card was in it was VERY hard to get anything to boot even once with the 5870.
On the Asus mobo everything works fine when it works with the sound card.
I haven't tried it with the optical drives unplugged yet. I assume to do that I would need to do both the SATA and the power cables.
I don't know how clear I was in the other post about this but I'll try to reiterate one thing that I found odd.
I am out of town until early Wednesday. Before I left I powered off everything. For the previous 5 or 6 days it had worked fine, but I had never turned it off at the PSU only the button on the front of the case.
I turned it off at the PSU and Decided to cycle the power to see if I could get it to boot. It didn't after I tried several times.
It had acted the same the time before when I turned it off at the back. But booted fine for some time after i let it sit for 5 or 6 hours.
Tried to make that clear but I am better with numbers than words
I will see if this is replicated when I get back home Wednesday.
I'd peek into the BIOS and see what your motherboard is reporting for voltages on the 3.3v, 5v and 12v source. Keep a record of those values during your testing.
If you can continue to get multiple successful boots at the simplified condition you can try adding back one more item at a time and see if you can detect if any specific part is causing a problem.
I just got back and it powered back on fine. checked in the Bios and this is what was reported as the actual values
3.3V was 3.37V
5V was 5.14V
12V was fluctuating a little in the lower 12's the other two were stable.
I wrote the numbers down to see if i could replicate booting without having done anything and after time turning the power on and off to try to get it to boot i achieved it again. Some of the other times The light went on on the mobo that has gone on before. and twice the fan spun at 100% on the video card and I got a long beep and three short beeps, 'no vga detected'
currently though my computer is on and running fine but it is only booting about 1/4th to 1/5th of the time I would estimate.
Update. It wont boot again this morning. every light turns on on all the hardware (everything was in as it was working fine yesterday) all the fans spin and it acts fine until the video card led on the mobo goes on and stays on. this happened multiple times before I stopped trying to get it to boot up.
I think it's time to test with another PSU.
It could be an intermittent condition with the PSU that's causing an undervoltage condition on the +12V rail that the video card doesn't like. HD 5780 might be more sensitive to that than a 9400GT, or that the 9400GT gets all it's power from the PCI-e motherboard slot.
A high quality 550W PSU should be plenty for a single HD 5870.
Yes the 9400 does not have a connector to the power supply so I assume it must get it all through the Mobo. Sadly I dont have another PSU at my disposal at the moment. Small college campuses don't overflow with custom built desktops, so I will get a new one and pray that solves the problem. Thank you so much for your help I'll report back my findings
I don't know if this is your problem but it sounds just like what I went through. Some days would be great, other days I would get the video beeping noise and it wouldn't boot. A few days later my monitor went out. I RMA'd it, they replaced the screen and tada, no more problems.
I havent gotten any issues from the monitors like weird stripes or the like, and having them plugged in or unplugged seems to have no effect on the booting. I am getting my hands on a PSU to test tomorrow actually, if all goes as planned, and it i can boot it up immediately all should be well.
Used a different PSU and its booted fine all day. Hopefully I am not speaking too soon but I think it was indeed the PSU.
I Know and old PSU can hold huge charges in them, so what do I do with the old one in terms of after care. Should I keep it, toss it in a dumpster, or does it have to be dealt with specially like youre supposed to do with batteries.