I just replaced a pair this week because of a similar problem. I determined that the problem was up at the headphone end, though, making the left audio channel cut out. There was one three-core wire running to a circuit board in the left headphone, and connection passing this signal along to the right phone. There was something wrong with the left channel wire, because I could jiggle it at that end to make the left channel kick in. I cut a length of the cable off and resoldered all connections, and still had the same problem.
The problem with Bluetooth as far as I can see is that you can listen to music fine, but if you want to watch anything visual, you get synchronization issues, where the audio lags behind the picture unless you buy headphones and transmitters with the more expensive aptX low latency technology (about £60 minimum for the headphones, when I bought a pair about six months ago for watching television).