Xbox was fine at 1440p 120Hz, suddenly refuses to work at 1440p 120Hz, getting a black...

My xbox has been set to 1440p @ 120Hz on my monitor for the past year. The other day, it started black-screening for a few seconds at a time while I was playing, about every 20 minutes or so (just random,
no uniform timing between them). Everything else ran fine, with no evidence of loss of graphics quality or anything like that... still booted up perfectly fine too, was ONLY getting a couple of seconds of no visuals.

I did a Hard Reset as advised by those in the online forums, which resulted in the Xbox booting up to a black screen. I could hear the menus through my headset, but nothing was being displayed on screen.
I took the following steps, in this order:

• I switched the HDMI cable, still got a black screen.
  
• Switched to a 1080p TV (resolution booted to 640x480 for some reason) with the new cable, and got constant on/off visuals when changing to 1080p resolution.
  
• Switched HDMI cables AGAIN to a brand new one supplied with my HDMI 2.0 monitor (pretty sure this is an HDMI 2.0 cable), set it to 1080p on the TV, ran absolutely fine on the 1080p TV (60Hz)
  
• Swapped displays to the original monitor with the new cable, turned it to 1440p (seemed to work), but got perma-black screen when selecting 120Hz.
  
• Swapped to my second identical monitor (which I bought a month ago) and repeated the above process, exactly the same issue.
  
• Did a troubleshooting set-up, booted into 1080p and changed to 1440p at 60Hz, resulting in a much more frequent black-screen issue than previously experienced, but not permanent.
  
• Dialled it down to 1080p again, back on the original cable (which has handled 1440p 120Hz for a while), and it works fine (for now).
  

I know this isn't the monitors, as one is a year old, the other is brand new, and they are both exhibiting the same behaviour. Why 120Hz mode would suddenly stop working makes absolutely no sense to me, and
neither does the fact that it ran perfectly fine a week ago, started going black for a couple of seconds at a time a few days ago (but showing no other graphical issues), then just refused to output any sort of signal whatsoever!

I am desperate to know if someone else is having this issue, and whether or not this is a sign of the GPU dying. I do not want to go without an Xbox for the next month and a half, but also don't want to spend
anything on a pre-owned xbox for ONE MONTH. If I have to, I'll play on a 1080p monitor just so I don't overload the GPU.

Any ideas as to whether this is a GPU, HDD, HDMI Chip, Software or "some unknown gremlin" problem? A fix would be appreciated! I've ordered an HDMI 2.1 cable just to guarantee that I'm not drastically unlucky
with the ones I have chosen, but I need advice regardless!

:)