Please try the following troubleshooting tips:
- Adjust the video playback settings within the app: visit the app's "Settings" menu (the small gear icon on the top right of the nav bar) and enable the "Play Video Over WiFi Only" setting
- Disconnect other devices from your network
- Clear the UMC app's cache by visiting Settings > Applications > Manage Applications > All > UMC and press "clear cache"
- Delete and then reinstall the UMC app