Just another update:
I did go back to the current version, 9.2, and it seemed fine with an old project that had previously been problematic with playback, but when I began a new project, the choppiness returned, albeit with 4k footage. Now, before you say it’s probably about my system settings etc…I subsequently rolled back to 9.0, and it’s all fine, now. I also reinstalled the latest Cuda driver (7.5.25) and it’s not causing any problems with 9.0 (and when I had it removed, 9.2 was still problematic).
So, the gist of all this is that, for some reason, Premiere CC 2015 9.2 has something buggy that’s causing it to not run properly on my system. I’ll wait and see how 9.3 turns out before upgrading again.