XPS 8940, occasionally not booting up

I should also mention I tried different monitors, keyboards and mouse, and that didn't help. 

Try rolling back to an older driver, also when you replaced the CMOS battery did you leave the battery out of the system for 15 minutes in order to reset the BIOS?

@JOcean​ It was doing this before I replaced the battery. I read the battery should be changed within 5 min. to avoid problems with the BIOs. 

If you replace battery within ~5 min, BIOS isn't getting reset. That's exactly not what you want to do.  You want to reset BIOS as @JOcean already said.

Remove the battery and press/hold PC's power button for ~30 sec before reinstalling the battery. Then see if it boots correctly.

There have been other XPS 8940 threads with similar issues that were only resolved by replacing the motherboard...

@RoHe​ It boots correctly most of the time. The problem only happens occasionally.

On two occasions when I couldn't get it to boot after several tries, I tried moving the computer to the kitchen where we have a more updated electrical outlet and it booted. So I'm wondering if it could be a power issue?

At any rate, here's what came up with the Intel processor diagnostic test

@strux​ 

So, works fine in the kitchen, right.... do it enough to be sure that it is OK there, or just one or two times...

So, this leads to what is different?

Not sure what this means, "where we have a more updated electrical outlet"? More wattage available on that line? Grounded line/socket?

This begs the questions:

1. Are you using a strip power/surge protector?
2. In the kitchen, did you connect everything you had at the old location and power those up too?
3. If there is insufficient A/C power wattage available in the old location, did you unplug all the other 'stuff' on that A/C line?
4. Did this start from Day 1 or sometime later?
5. When it doesn't boot, color of the power button and the PSU led on the back?

However, at first thought, I suspect an A/C power supply to the PC issue. Bad socket maybe, have you tried others in that room and see if it boots?

@ispalten​ I had the problem when I first booted up after changing the CMOS battery and adding my new RAM, but haven't had the problem since.

We have an old house and the computer is plugged into a strip, which is plugged into a 3-prong converter, which is then plugged into the outlet. Bad, I know, but . . . 

The kitchen outlets were updated years ago, so they would be more reliable. 

It could just be that time it took to move everything to the kitchen, allowed the computer to cool down. I had the same problem a couple of weeks ago, and just letting it sit for awhile (without moving it) seemed to work then, too. So maybe a cooling fan problem? 

@strux  - Maybe you really need to get an electrician to check out that outlet where PC has problems, sooner than later...

@RoHe​ I haven't had the problem since changing the CMOS battery with the one exception of right after changing the battery - the first time I turned it on got the black screen. But hasn't happened since then. 

@strux​ That black screen at the first boot is not unusual with the battery replacement.

@JOcean​ Of course no sooner did I say it hasn't happened, it happened again.

Guess I'll try taking out the battery again and waiting 15 min. this time. 

Seems to happen most after I've done a lot of work on Lightroom & Photoshop, then do a restart. 

After you've done a lot of work on Lightroom & Photoshop, what happens if you do normal shutdown and boot, rather than a Restart?

Have you tried changing the Fast Startup setting in Windows (not Fast Start in BIOS)?

1. Boot to desktop
2. Open Windows Power & Sleep screen
3. Click Additional power settings
4. Click Change what power buttons do
5. Click Change settings that are currently unavailable
6. UNcheck Turn on fast startup
7. Click Save changes
8. Do a full shutdown, not just a restart.
9. Power on and boot (Note: boot times will be longer with Fast Startup disabled)
10. Do your Lightroom & Photoshop work etc as usual, and then do a restart.
11. Does it fail now?

@RoHe​ I'll try those things, but the problem is so random, it only happens maybe every couple of weeks, now. So it's hard to tell what works.

Yesterday I worked on photos for hours with no problem. This morning I start up Lightroom and it's really slow and I can hear the fan running louder. I can't blame over-heating this time. 

@RoHe​ I tried removing the CMOS battery again, only this time I left it out for 15 min., and now everything is running amazingly smooth and snappy. Hopefully that also resolved the boot up issue. (I'm thinking it was booting up correctly, just not showing up on the monitor, but we'll see).