I wonder if anyone here could help me trouble shoot a motherboard/ bios problem I'm having combined with the gtx 750 single fan ( 02g-p4-3751-kr)

After installing my new gtx card without much trouble it worked, but only a few days in Dell Studio XPS 8100 running win10 64 bit
The PSU in the dell is only rated at 350w but the card i replaced had a slightly higher power consumption so i reckoned that the gtx card should be ok.
The dell only has a pci-e 2.0 slot, but then again this is backwards compatible. I had the card working, then updated drivers, and carried on using the pc as normal for a day or so.

My problem started when after a restart i could not get passed POST and got 6 beeps indicating a graphics problem. I tried this new card in another pc where it worked fine, running well at a stress test for over an hour, so the card is fine.
Rechecking my dell with my old card work fine as well, so I believe the mother board to also be ok.
To trouble shoot further, I have just gotten a new PSU, 500W with 38A available for the 12v bus, but this also makes no difference.
To finish I've cleared the cmos and checked I have the latest bios from Dell, this however is quite old, last updated in november 2011.

I'm, now at a loss what to do next. Should i try to replace the motherboard as well or just give up on trying to get this aged dell to work with a 4 k screen.

So far I bought the card, a psu and a new 27 inch 4k screen, so spend a bit of money
I could still return it all, but that would get me back to my old tired setup

Any thoughts or sugestions welcome

Thanks for your suggestion. Would you know if this also supports a 4k monitor. I notice it does not have a display port.