It seems my post was unclear again, sorry. Please allow me to rephrase.Haven't tried copying all the music from the NAS locally but that wouldn't be practical, since I wouldn't be interested in such a solution, so I won't explore it.
The issue you're experiencing is unexpected and undesirable. It would be great to figure out what is causing the issue, so that it can be eliminated in some way or other (code changes, configuration changes etc. - whatever is necessary/appropriate). I suggested the copy-to-local test as a way of gathering further information about the issue, not as a solution. If you won't perform tests like this then we won't find the cause of the issue.