Hi,
Adding a GPX layer should not make the application unstable but it can impact very hardly the loading time of your Tour.
If you have added your GPX to the webmap through ArcGIS.com viewer, the track is now included in the webmap through JSON.
Depending on your track resolution, it can be quite heavy.
Map Tour has an on purpose aggressive timeout to force author to use lightweight content.
Please try to simplify your track using ArcGIS Desktop or a free tool like GPX Editor and let me know if it doesn't get better.