System freeze (Nvidia / Flash)

[Thomas F. Duellmann]

Hello,

I'm experiencing very strange system freezes for some time now. 
Occasionally (at least once a day) my system freezes while (or after)
using flash content (e.g. watching youtube videos). The whole UI freezes
(even the clock) and the only thing I can do is moving the mouse
pointer. Any keyboard inputs do not have any effect. The system itself
still seemed to run (because I could log on via ssh on that system).
Finally the only thing to get it back to work is a hard reset.

Unfortunately the only hint I got yet is a kern.log entry:

NVRM: Xid (0000:01:00): 31, Ch 00000007, engmask 00000120, intr 10000000

(which is the same after each crash)

I found out that this entry has to do something with nVidia drivers, so
I'll provide some information on my GPU and the corresponding driver:
GPU: Nvidia GeForce GTS450 
Driver Version: 290.10 (installed manually from Nvidia website).

While trying to get rid of this problem I tried the recommended Nvidia
driver provided by the "additional drivers" tool and several other
manually installed drivers (like 285.X) - but the problem persisted.

I also tried to use different Adobe Flash (Player) versions from Ubuntu
repositories or Adobe website - but that also didn't have any effect.

I'm trying to find a solution for this for some time now. Unfortunately
the only alternative I currently see is reinstalling my system - because
it is very annoying and disrupting.

Thanks for your help in advance.

Thomas