GNOME Shell Graphical Corruption and Freeze

I need some information from those who are using GNOME Shell with
Trisquel 5.5:
Do you get graphical corruptions when you use Shell?
Does your Shell sometimes freeze and then you have to do a hard
reset/shutdown?
If you do get those problems, what hardware are you using?
Is there anyone using Shell on one of ThinkPenguin's computers?

A brief description of what I mean with graphical corruption:
* Some letters are replaced with boxes (this happens only when Overview
mode is active)
* Some rounded corners (like the notification popup) turn to solid boxes
* Sometimes the window previews in Overview mode are missing
* Big patches of solid colour (I get different shades of blue)
* Some UI elements change to patches with lots of rainbow coloured blocks
* Overview mode activates and then I can see some corruption (like
missing windows) and then when I switch back to my desktop the UI
elements start to flash repeatedly

Those are only some of the issues I can remember right now. In all those
cases if I press ALT+F2 and type 'r' I can restart Shell and the
corruption goes away. But usually when the corruption starts I will get
it multiple times again after I restarted Shell. Eventually Shell will
just freeze.

The corruptions seem to occur sooner and more frequently when I have
more applications open. But this observation might just be coincidental.

When Shell freezes I can still move the mouse but nothing else responds:
I cannot activate Overview, I cannot even open a terminal with the
keyboard. If I have Exaile open when this happens the music continues to
play but my laptop shortcut buttons cannot control playback anymore.

Sometimes the keyboard does work and I can switch to a different Virtual
Terminal to restart the computer. But most of the times I have to do a
hard shutdown.

I have not yet investigated this in much detail and I'm not sure what to
look for, but when I have time I will look into it. I had to switch back
to GNOME Classic now because this has become too annoying and I'm
worried that I could damage my hard drive from all these forced shutdowns.

I am having these problems on a Laptop with an Nvidia Quadro NVS 140M
graphics card. I am obviously using the Nouveau driver which I thought
might have some problems. But from what I've read elsewhere people with
the "open source" ATI, proprietary ATI and proprietary Nvidia drivers
are also having some graphical corruptions and freezes. I don't remember
if I read about any Intel graphics users complaining.

For the record, I have this problem with the default Trisquel 5.5
repository, with and without the latest linux-libre kernel (3.3.6) and
even when I updated to Shell 3.4 (from the ricotz testing PPA). So right
now I am not sure if this is related to the hardware drivers or if there
is something wrong with Shell in some cases.

--
Morne Alberts