Video choppy after exporting - Need help!

Hi All,

I've edited my video in Premier Pro. The replay in the application is smooth. When I export the video it's choppy. I've been through the suggestions in the community here, however, everything I've tried failed. I've recorded the source videos on my iPhone (4k, 60 fps) and thought this might be the issue. I've also tried the same source material in other editors and did not have the same choppy output.

Grateful for your suggestions how to solve this as I'd rather not bin this project and start again!