I found that Flash Player 18.104.22.168 Beta leaks memory badly. No such issue with 22.214.171.124 Beta. Reproducible with clean browser builds (Beta and Nightly tested only in repeat testing).
I was watching Rio Olympics live streams with 126.96.36.199, RAM usage was fairly steady. With 188.8.131.52, RAM steadily increased to 100% in about 30 minutes of steady streaming, completely used up the virtual memory until Windows started to shut down the browser. Problem disappears after rolling back to 184.108.40.206 Beta.
Flash Player version 220.127.116.11 (Released: 24-Aug-2016) appeared to have resolved the memory leaks that was introduced in 18.104.22.168. All is good.