I've been running 64-bit Ubuntu for about 2 years now, but a few days ago it developed serious problems. Whenever I would load a Flash page* the computer would crash... well, sort of. The screen would go blank immediately, eventually causing the monitor to give the "no video detected" message. The system would remain powered on, but it seems that it's not really running. If audio was playing at the time it would continue for 5-10 seconds after the "crash", but only repeating a ~1 second loop of audio.

I backed up my Chrome user profile, uninstalled Chrome (I was using the beta version), and reinstalled Chrome (the stable this time); this didn't help. I removed the libflashplayer.so from the Firefox plugins directory and replaced it; now Flash won't work in firefox at all, but at least the computer doesn't crash. I may have had flashplugin-installer or flashplugin-nonfree installed at some point while trying to make Flash work in the first place, so those (and others) are possible points of interference.

Any ideas?

* This always happens in Chrome, which uses its own Flash player, and about 50% of the time in Firefox, which uses Adobe's Square flash player for 64-bit systems.