As I remember.... that switch is attached to the lever mount. The switch comes with generously proportioned wires, and attaching these to the rear of the switch leaves several LARGE globs of solder. Those globs protrude far enough into the mounting area that an insulator is needed to keep the attached wires from touching something inside the pocket where the switch sits. 

The reason I remember this repair is that in my repair, I ended up removing all the large wires from the new switch and replacing them with smaller 16ga automotive wire simply to get the soldered connections small enough to be installed into the switch housing without problems. The reality was that the switches made for Spares seemed to be made differently from switches made for Production. The parts are the same, but something about the manufacturing technique was different. 

A switch that is/was contacting the housing through its rear contacts, OR a switch installed without the insulator (thus allowing contact) could be your issue. Intermittent electrical contact made by the those terminals touching the lever mount would probably cause the issues you are having. 

If my memory serves me well, the horn is always hot and the button provides the path to ground. It is probably depicted on the factory wiring diagram.

I had the same problem with my /5 horn.  I wrote up my solution on the Vintage BMW Motorcycle Owners site, an easy fix if you have the same fault:

horn inop on /5 - grounding problem - Vintage BMW Motorcycle Owners

Thanks for the help! I'll try these out.