The temps are fine, I've had this pc for 18 months and it's just started happening recently do you know what it could be or not?
What psu would you recommend anyway?
The PSU could be breaking down or it could be causing damage. This is real junk-tier when talking about a PC full of quality components. It's full of dire third-tier Chinese capacitors and over at JohnnyGuru, Oklahoma Wolf found some
pretty horrifying crossload results. You never "know" what the problem is in a situation like this, you can only guess from most likely to least likely. If your temperatures are fine, the PSU is the next culprit even if you have a decent one, which you most certainly do not!
That your PC may have other damage to it as a result of the PSU is something that cannot be dismissed at this point, given how long you were "protecting" your components with it.
Generally speaking, the least expensive PSU worth buying tends to be the modern Corsair CX series (so no green letters on the unit, which signifies and older, much inferior platform). With a 1080 Ti and an 8700k to protect, I'd go at
least with a Corsair RMx, SeaSonic Focus, or EVGA G2/G3/GS (there are other good ones, but these are solid PSUs that are the easiest to find).