It sounds like something I've had, on and off, for years. I find it mainly occurs when I'm going downhill. Like you I find that bringing the arm in front of the body helps - sometimes I rest it on the waist-bag I carry, or just dangle it like a weight, for a bit.
I consulted a physio, who diagnosed weakness in the muscles under the shoulder blades, and a series of stretches and exercises. They seem to have helped a bit. But if you find out anything more definite, I'd be interested to know! The physio seemed to think it was connected to my tendency to hunch my shoulders when concentrating, leading to face ache, tension headaches and migraines. Can't say these have improved much.