First thing to not do is turn off the 'mains' to re-set the PC. Just hold in the on/off switch on the PC to safely shut down.
Lines on screen indicate, to me, that the GPU card is failing or not getting enough power. Ram not being seen fully could also be not getting enough power. Stress testing the CPU and GPU individually really means nothing as the PSU isn't working hard enough.
90c for card is too high so internals need dust removal.
In my humble opinion it could be the PSU either failing or not up to the job.
Internals need to be cleaned of dust build-up.