You should be on FFMPEG 3.x or greater for the best performance.
Try these settings
Generally a lot can be cleared up when using TCP instead of UDP. Also by increasing the duration to wait for data we can allow the camera a chance to complete a stream from long distances or poor connections.
RTSP Transport : TCP
Analyzation Duration : 1000000
Probe Size : 1000000
Got an RTSP camera?
See How I optimized my RTSP camera settings in Shinobi.