4K (and 2K) playback stutter issues with Mac Premiere Pro 2018, worst on DPX

I'm really struggling with this issue and have been for months, and no amount of upgrades to my system seem to attack the root issue, which is that I get very stuttery playback, even from SSD (both data and boot drives; data drive is on USB 3.1 bus) after a few seconds, every time I hit play in a timeline, if I am using 2K or 4K DPX files, and also 4K ProRes files. Indeed, the problem has periodically manifested on 2K ProRes, as well.

The system is a tricked-out 12-core 2010 Mac Pro:

2 x 3.33 Ghz

64 GB 1333 MHz RAM

500 GB SSD boot drive

NVIDIA GTX 680 GPU

There has been no appreciable difference when using a different/older NVIDIA GPU (since sold off).

I have tried every configuration of settings adjustment I could think of in terms of changing caches and buffer size, playback resolution, all of it. Nothing seems to change the issue. Indeed, as I saw alluded to on a different thread, lowering the playback resolution seems to actually WORSEN the stutter, in some cases.

Main question at this point is: can anyone see an obvious brick wall or bottleneck here? Is it worth trying a GPU such as the 1080 Ti (at great expense)? Would I be better off building a PC from scratch and abandoning Mac altogether? (I am not going to get into the madness of the iMac Pro.) Any insight or suggestions appreciated.