Flash player works only for a while

[gruntly-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org (Theodore Ruegsegger)]

I've been annoyed for a while now by a consistent, reproducible
problem with the shockwave flash plugin to Firefox.
I don't know when it started because it's only been in the last coupla
months or so that I've been playing a lot of flash videos.

The flash player works fine for some time after I start up Firefox. If
I leave the browser running and return to it some time later, flash
doesn't play properly; the picture jerks or freezes and the sound
comes out like a random syllable repeated endlessly, all this while
the progress bar moves along as if everything were fine. If I exit
Firefox, make sure it's terminated, and then restart it, flash plays
fine again.

My plug-ins list says I have Shockwave Flash 10.0 r32. My Firefox is
version 3.5.3 which calls itself Shiretoko. Platform is Kubuntu/9.04
(jaunty).

FWIW, I've noticed the same problem with Firefox version 3.0.14.

I've tried googling words like "flash stops working" and got lots of
hits about the flash player not working at all, which isn't my
problem. My player works fine and then deteriorates after the
browser's been running for at least a few hours.

Am I the only one with this problem? Can anyone give me suggestions on
how to go about solving it?

Also, how good are the current Free flash players, like Gnash? Can I
easily remove this proprietary one to install it?

Ted