Chosen Solution
I’m having an issue with my late 2013 Imac. For the last week or so it has been randomly freezing up. The display will just freeze up leaving an odd fragments of the screen visible. All I can do is hold down the power button and shut down then restart. Downloaded and installed the OS X El Capitan, ran the Apple Hardware test, evrything OK. Any other ideas where to troubleshoot? The graphics is NVIDIA GeForce GT 755M 1024 MB 3.2 GHz Intel Core i5
Sadly you have a bad GPU. See if Apple will cover it under the extended warranty.
