Stuttering Playback after video card upgrade

Hello! I have a Mac pro tower 2009 (4,1) running 10.11.6 with 24gb RAM, and until recently I had 2 of the stock Nvidia GT120 graphics cards in it, each running a monitor. I would have my timeline and controls on one monitor, and display the video fullscreen on the other monitor using Mercury Transmit. This was working perfectly.

I then removed the GT120s, and replaced them with a single GTX680 4gb graphics card. Now my footage will not play back smoothly on the second screen when in fullscreen mode, and I can't for the life of me figure it out.

Some things I should note:

- All sorts of different footage has been tested, including low data rate, low resolution footage, running off an SSD, and it makes no difference at all.

- Video plays fine on the second monitor when in non-fullscreen (bordered) mode. As soon as I enter fullscreen mode or enable Mercury Transmit, playback suffers.

- I have tried lowering the operating resolution of the second monitor from 1080 to 720, and it makes no difference at all.

- I have the latest nvidia drivers for the card, as well as the latest CUDA drivers installed and running. Premiere is up to date.

- When using CUDA under project settings, I get the worst performance. OpenCL yields slightly better results, with software-only somewhere in between.

- When making proxies in Media Encoder, CUDA yields the fastest results, followed by OpenCL, with software-only being significantly slower.

It occurred to me to try putting one of the GT120's back in, and using it to drive the second monitor, but I understand this may introduce more problems than it will solve. Also, the GTX680 is significantly more powerful than the GT120, and I'd rather figure out whats wrong than use a stop gap measure.

Can anyone offer some suggestions as to why this may be happening or how to fix it? I've done quite a bit of searching, but I'm hoping I'm missing something obvious.

Thanks!