I've been having this problem when viewing online videos/tv, primarily channel4.com/4od and similar sites.

The videos load up and play with no problems, but if the view is shifted away from the video (e.g. scrolling down the page so the video is no longer visible or switching to another tab or window), after a little while (up to a few minutes) the audio from the video stops and, upon returning to the video window, the video has been replaced with a light grey rectangle. The problem seems to be different from others described here since the videos themselves are running with no problem until the view shifts away from them. Flash-based chat applications have the same problem.

I'm running Firefox 3.6.15 in Ubuntu 10.10 (Maverick). All systems updated repeatedly, as far as I know the problem has existed forever. The issue doesn't seem to exist in Chromium, interestingly (but I prefer Firefox...)

Any suggestions on diagnostics to try to find the cause and solution?

The computer itself is a Lenovo G565 AMD Phenom 64 tri-core laptop, 4GB memory, Mobility Radeon HD 5xxx. It should be able to handle a video in a browser!

I tried two different systems running FF 3.6.15 and they both worked without problems. I tried it on youtube vids.

What level of flashplayer have you got installed?

On Mandy I have Shockwave Flash 10.1 r82 installed. Works fine. This system has Nvidia hardware. Its possible its your video driver at fault.

The other system was Ubuntu 10.04 and FF 3.6.15.

Run Firefox from the terminal and look if they are any error-messages displayed when this issue occurs.

Sorry, been a while since I've had time to have a look at anything. Anyway, whole bunch of error messages:

(npviewer.bin:2486): Gdk-WARNING **: XID collision, trouble ahead

*** NSPlugin Viewer *** WARNING: unhandled variable 18 (<unknown variable>) in NPN_GetValue()

*** NSPlugin Wrapper *** ERROR: NPP_Destroy() wait for reply: Connection reset by peer

*** NSPlugin Wrapper *** WARNING/build/buildd/nspluginwrapper-1.2.2/src/npw-wrapper.c:1923):invoke_NPP_SetWindow: assertion failed: (rpc_method_invoke_possible(plugin->connection))

The first two started as soon as the video started playing and were repeated over and over, the second two came when it vanished. I know npviewer.bin is the flash plugin for firefox, so seems related. Judging by what I've come across in google, this could be something to do with 64 bit wrapper?

As for the video driver explanation, there are no problems working in Chromium or any other video problems at all on the computer, so it seems more likely to be something firefox/flash specific to me.

The version of Flash is 10.2 r152 and is up to date, according to FF. As a final note, no matter how hard I try I can't seem to replicate the error on Youtube.

May be you should try the 64-bit viewer instead of the 32-bit with wrapper.