What program are you using to play back the video?
Sounds like it could be a mismatch of Video Mode FPS to source video FPS.
You can check the project’s video mode by clicking on Output, then click on Properties. The FPS should match your source video, which you said that it’s been converted to 60 FPS.
If it doesn’t match, you can change your Video Mode in Settings.
When you get back to export, click Reset, then try exporting again.
