try and reseat the GPU ( just in case poor contact ).
change out the cable you use for another one ( to eliminate the cable )
did you do a fresh install when you changed the GPU's?
did you delete the old drivers before instaling the new GPU whit nvidia drivers?
this have nothing to do whit OC or memory, the atifacts is made by the GPU, or cable+++
try and install an older driver, maby the 355.82 ( if you can find an old one ) and try and see if there is enny diffrence then.
and do a custom instal, do not instal the nvidia experience.
glad is worked out for you smegmer.
no need to update to newest update all the time 😛
i can spend months on the same GPU driver for example before i update it ( this can come down to the game i have for example ).
no need to update some thing that works 😉