I found that Flash Player 22.214.171.124 Beta leaks memory badly. No such issue with 126.96.36.199 Beta. Reproducible with clean browser builds (Beta and Nightly tested only in repeat testing).
I was watching Rio Olympics live streams with 188.8.131.52, RAM usage was fairly steady. With 184.108.40.206, 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 220.127.116.11 Beta.
Flash Player version 18.104.22.168 (Released: 24-Aug-2016) appeared to have resolved the memory leaks that was introduced in 22.214.171.124. All is good.