I was pointed towards the video levels being the cause, and digging deeper I found the new release of 177.66 driver for nVidia:
This allows you in the Control Panel to switch between 0-255 (PC levels) and 16-235 (video levels) modes. After updating the driver the picture got more contrast and the random contrast change nearly dissapeared. I say "nearly" due to some odd re-occurences. But overall, I can say is nearly gone. Just installed it 2 hours ago. The black pillars (on a 4:3 picture) used to be badly pixelated, now they are not, and it also appears the colour gradient display improved a bit (smoother transition from one shade to another, I used to get lots of concentric circles... you know what I mean).
So far, the only downside I noticed is some "low frame" effect on TV3 only, even if the cpu load is the same as before (45-50% on TV3... I know... I do not have HA... I did not manage to get it working...). I hope some future playing with the codecs and some tweaking will sort it out, I had this happening before. Maybe it is not even related with the driver update...
Hopefully other users out there are looking for an easy way to get the video levels changed.
Please post your feedback regarding the new driver.
NOTE: after installation, it was set to 16-235 by default, I had to change it to 0-255.