Have you tried updating or rolling back the video driver?
Moving to the hardware forum.
That was certainly worth trying. Rolling back from the 2015 drive to the 2013 driver gave a slight improvement, but still not as good as things were with my older card. One new thing I'm now observing is that an h.264 export directly out of Premiere is using nearly 100% of my computers's CPU. When the GTX 460 was in place, it used to be about 50% when exporting from Premiere (AME always used 100% of the computer's CPU).
This may be because CS 5.5 does not properly support any NVidia GPU that's newer than the old Fermi architecture (which is what the GTX 460 is). But the GTX 760 is of the newer Kepler architecture, which uses a different CUDA compute capability version (3.x) than in Fermi (2.x). CS 5.5 does not properly support CUDA compute capability higher than version 2.x.