Move your cache from the hard drive to the USB stick. Don’t copy it – move it.
When you keep your cache on the hard drive, the same one is used for customs/TU4 play and online/TU5 play. That gets messy fast.
I echo Nyx’s comment 100%… I had /exactly/ the same problem you are describing and moving the cache fixed it completely.
Online mode: USB stick contains song cache and Title Update #5
Customs mode: stick contains song cache, Title Update #4, and all custom songs
Works like a charm.
You are about to leave Rhythm Gaming World and access the external songs DB. While no song or file sold for commercially available videogames is hosted there and only fan generated content is allowed, Rhythm Gaming World has no direct control over the database and no files are hosted here. Alright, we had to made you aware of this, now you can visit the database!