sounds like your phone have bad memory management. I used to have that problem as well, song stops playing (sometimes even in the middle) and nothing, not even playing next song.
Either way, my current favorite is BlackPlayer, great interface. But I also recommend PowerAMP for excellent sound, and Stellio (my previous favorite).
Linkme: blackplayer, poweramp
(stellio isn't on play store, it's on their site)
I also had bought Stellio and pretty much ALL of their themes. I love that player, still use it, even though it has it's quirks like only showing lyrics in Cyrillic text more often than not. No, what I want to say is, Stellio is back on the Play Store! If you bought the Unlocker, just get in touch with them on their Official contact page, refer to your Play Store Order ID and you should get your unlock code for the app and/or the themes.
Also, Stellio did this well when I tried it sometime ago: http://stellio.ru/en/
However, it's connected to a phone-side music player. I couldn't figure out how to use the watch-side app with my music player of choice (Google Play Music).
(Just remembered what it was called.)