Remember to check that your playback problems (mysterious fadings and mutings) are not originating from some input effect. I spend quite a lot of time figuring out why I had playback problems only because the template (piano+vocals) included some strange input effect in the vocals channel. I even thought the problems was my SSD (there’s a knowledge base article about Solid State Drives causing playback problems in some occasions), but the related settings were just fine. Then I thought the problem was some incompatibility problem with my system and the 64-bit version and reinstalled Cubase having still the same problem. 
I don’t know if the input effect, what ever rock thing it was, was working properly or not, but for such a beginner as I am it gave quite a shock.
I hope this tip saves somebody’s time. 