1. Don't ignore the pinned post in this forum (link in my .sig) about posting OBS Studio log from session with problem when asking for help/support
2. Having 'great wired Internet' is measured by most folks as download speed, which isn't really relevant to streaming. Even a high upload speed isn't enough if Jitter and/or Latency are an issue. In many locations, Internet bandwidth is NOT symmetric (ex. coax cable based connection common here in North America), and other traffic can consume upload bandwidth (ex video conferencing).
3. Meaning you should be doing real-time hardware resource utilization monitoring on your PC (Task Manager > Performance) as well as traffic monitoring on WAN connection. Without doing that, you can not have any idea where a bottleneck may be
And there could be the simple case of your PC being overloaded, sometimes by background processes (Operating System, other apps, especially sync apps like Google Drive, OneDrive, etc)