Apple randomly deleting music from iPhone - a possible solution
A question was raised earlier this year, with regards to music that was ripped from your own CDs, transferred to their iPhone, then a short while later (maybe a week or two) disappear! Any music that had been purchased via the iTunes Store still remained.
My solution to this conundrum was to go though all my music (on my laptop) that I wanted to have on my iPhone, then make each track a favourite (Star). Once I had done this, I selected in iTunes, View/Filter By and tick Only Favourites. Select all then drag over to your iPhone.
I have tried it with 1500 songs without any issues!
Hope this helps!