When I run I use an armband and put my LG/Android in the sleeve while I run. I don't like earbuds so I use normal headphones with a cord that goes into the smartphone. However, the sound will cut out and I have to keep fiddling with it to get it to work. This is regardless of whether or not the headphones are cheap or are expensive. Is there any reason why this happens? Weather isn't an issue either--it happens when it's hot or cold.
Some bluetooth headphone / smart phone or MP3 players "pair" better than others. I use a pair of Jabra earphones (which I love otherwise) but if I wear my phone in the back it can cut in and out. I'm surprised you have that happen with an armband but some of the types of bluetooth connections need the phone or player very close to the earphones and others seem to be more tolerant of the phone being in backpack or further away
sweat would be my guess. finds it way into the circuitry.