I have experienced that on occasion myself, but the problem was the assets. I replaced some terribly polygon-heavy splines with a similar variety but better, and the problem went away.
The issue could be how the spline is constructed, i.e. the segments and LOD setup, which unfortunately may not be end-user repairable. There could be other issues as well such as overly-detailed textures, and other stuff that Content Manager may not see. Content Manager has a big and better filter and catches a lot of stuff, but there are some stuff that still falls through the cracks and the program will then flag as faulty internally. The program will continue to load these assets, but they are problematic.
With that said my suggestion is to check the log while running the route. This means running in a window so you can do both at the same time. When in the route, go back to the Launcher. Click on Clear Log then Show Log. The clear step removes all the previous stuff you don't need, and the show is obvious. Click on the colored radio buttons to filter out what you don't need such as general information, aka black, and green for all is well. You want to leave the red and yellow.
With these enabled, problematic assets or other issues will have a red circle X next to the line. You can't pause the scrolling directly, but you can stop the scrolling by dragging the screen up. With the log stopped, you can then highlight, copy, and paste for later analysis. There's a request in to save the active log to disk as an advanced feature, but as you know some things take higher precedence than a feature request.
Using this log, I was able to track down assets that were shown as okay in Content Manager, but had problems anyway. By copying down the information, I researched the various assets and was able repair them. I did this initially in T:ANE and have found some I could address in TRS19 as well but haven't had a chance yet.
