Hey there, could you please read the orange box appearing above your post and add your hardware internet specs to your profile? This can help us a lot in helping you
For your actual problem my first idea would be trying the TCPRelay software one of our Forum Members "invented". Check this thread: viewtopic.php?f=70&t=7159&hilit=rtmp+forward
Seeing you had no bandwidth problems with other software this tool might be able to help you.
If you have more than one server location you can try to stream to, please also try another one and see if xsplit produces normal bitrates there. A server with a higher ping / or which is further away can still have a better connection (more constant).
All results can help the developers pinpoint your problem further, so don´t forget to report if it worked