Some trains are simply more sensitive to electrical connectivity and while one train may run fine, another may not. Part of the problem with seasonal layouts is that each time you disconnect the track pieces, it can slightly bend the pins and create connectivity issues and that gets repeated (and worse) over time. In addition, since the track is not permanently laid down, it tends to flex, especially on carpet, and that creates more issues.
I would start by testing with a DVM all around the layout to make sure you are getting sufficient voltage to all sections of the track. I would then put a train on it and test it again, with the train at different spots on the layout. That should give you an idea of whether you have any voltage loss locations that are causing the UP to run sporadically and you can repair those areas.
Look at some of the many threads on this Forum about how to deal with FT connection issues and for specific ways to enhance connectivity, such as this one ... Need advice on fastrack connection issues | O Gauge Railroading On Line Forum (ogaugerr.com)
