Look, I'm not trying to be rude here, but I think you are VERY, VERY confused about what the FPS value for your recordings means. There is nobody, anywhere, recording at 720fps. That's just not possible, on any consumer level hardware at any kind of watchable resolution, let alone 1080p. I don't think you fully understand the insane amount of resources that would be required for that kind of recording.
I have a 1080ti, and a 4790k overclocked to 5.0ghz on a fully custom water loop, and my PC can't even manage to peak over 600fps recording at 720p. This is the attempt to record at 720p 720fps:
https://pub.rachni.com/img/obs64_2017-06-29_13-27-58.png
Code:
13:28:05.784: Output 'adv_ffmpeg_output': Total encoded frames: 11984
13:28:05.784: Output 'adv_ffmpeg_output': Total drawn frames: 23831
13:28:05.784: Output 'adv_ffmpeg_output': Number of lagged frames due to rendering lag/stalls: 7278 (30.5%)
There is just
no way anyone is recording at 720fps. So please, show me where this 720fps recording is. I would love to see it! Their GAME might be running at 720fps, but you don't need to record at at a matching FPS to the game. You really, really, absolutely, need to understand how completely ridiculous what you're saying is.
So please,
please, set the FPS back to 60, use the recording presets (Settings -> Output, Simple output mode, change recording quality to Indistinguishable Quality, and then select NVENC as your encoder). Do a test, and post the log.
If it STILL DOES NOT WORK after you did that, we will have a baseline to work off and can do further testing from there. Until you provide the log following those steps, we cannot help you further.