I have been having terrible performance issues with Flash plugin for a very long time.
It crashed almost every time I try to play something. This was happening in Firefox, Chrome, Chromium, Konqueror, pretty much every browser.
I tried uninstalling, reinstalling, downgrade to older version, compile my own from source, and just about anything else that was recommended by the experts at Google search results.
I now have it "fixed". It only crashes about 50% of the time, unless I am playing an embedded video where it crashes over 75% of the time.
How did I accomplish this, you ask?
I installed Mozilla's experimental browser Aurora, and keep it up to date with nightly builds.
Who knew bleeding edge would be more stable than "stable"?