iPad loses songs

I'm using an iPad3 with iOS 8.1.2 and it loses songs. The iPad only contains songs I bought by iTunes. But the iPad deletes songs and I have to download them again. The missing songs were definitely not deleted by me and they was stored on the iPad (not cloud streaming).

Today I downloaded them again, free space was 2 GB. 6 hours later without using the iPad I have 5,3 GB free space and many songs are lost. It is possible to download them again, they are not on my iPad but in the cloud. Has anyone an idea how I can avoid this auto-deletes?

Thank You.