When you start a live stream, your bandwidth may fluctuate and get lower at certain times. Even if XSplit Gamecaster tested your initial bandwidth, it may still become generally lower after a period of time (e.g. when more people come online using same DSL trunk). If this happens, you will see lag or dropped video frames.
Note: We are working on a solution that can dynamically lower bandwidth use for streaming by detecting when your bandwidth is constrained and automatically restores the original streaming bandwidth when your connection improves. This feature will be included in a future update.