iTunes crashes if not running Caffeine

I have a MacBook Pro running on 10.10.3, 2.9GHz, 8GB, iTunes 12.1.2.27.

Since the Yosemite update 8/4/2015 had lots of trouble with iTunes, crashed and MacBook would not restart had to erase hard drive and start again. Since then all seems ok except iTunes will only keep playing if I use Caffeine to keep the screen alive. If not music stops and, sometimes have to press power button to wake screen, unlock with my password then sign back in on iCloud and restart iTunes. I have deleted the version of iTunes again and redownloaded. I have copied all the tracks from my older Mac, (which runs perfectly normally, its on Mavericks), not taken from Time Machine when I retrieved my files. Sorry if I am not making myself very clear, not very au fait with technical words.