Mid-2010s Kernel Panic GPU - best options?
Suggested problem tracing /and repair) steps (after each test the performances).


Vacuum clean the inside of your Mac to improve cooling.
Google and Install gfxCardStatus, Leave on Dynamic Switching. From there it will Monitor and Alert you to what apps switch you to the Nvidia Graphic Card.
Also open the Activity monitor - Energy tab, where you can see when the Nvida card kicks in, and does it switch on/off irregularly.
Monitor your Console log for irregular (fast)GPU switching.
Do some hardware testing: 



  GpuTest is a cross-platform (Windows, Linux and Max OS X) GPU stress
  test and OpenGL benchmark. GpuTest comes with several GPU tests
  including some popular ones from Windows'world (FurMark or TessMark).


This test might help you decide to replace the card.