iphone - music stops frequently

Hi guys,

Went to premium so I could use it on my iphone, I then spent a couple of hours moving all the playlists to 'offline' but I encountered the following:
- some of the tracks stop every few seconds for a couple of seconds, which is very frustrating. One playlist seems to work perfectly but others dont. It is worth mentioning that the ones that dont, I didnt download all in one go (I was getting the hang of it !) so is this the problem, and if so how can I 're-download' ?
- the music also stops when I try to use my iphone for something else (unlike with itunes)

I then checked itunes on my iphone which worked perfectly so its not the iphones capability to play music which is the problem.

Please let me know how I can get the music playing without stoppages.

Many thanks