How do you KNOW your Internet is fine? using download traffic patterns to claim outbound traffic should work.. is not a valid thought process
Hopefully you aren't using WiFi, subject to all kinds of interference and bottlenecks which most services (but not livestreaming) have buffers to mask/overcome
What are you doing for real-time hardware resource monitoring on your PC and LAN, including firewall/router? With monitoring, you most likely find your reason...
I'd advise starting with making sure your PC is actually capable of sustaining volume outbound traffic at a sustained rate above your streaming rate (upload large file to something like OneDrive, Google Drive, etc). You could have any number of issues on your PC causing a problem. Or someone/something else on LAN using upload band witch, etc
the list of possible causes (none having to do with OBS) is long