Ok I think I figured out the problem (well I think ! )
It is the overheating of the laptop that is causing all these BSODs.
1. Exchanged RAMs ( I bought 2 new kingston ones), still BSODs.
however I noticed that after being shut off for a while, TX2Z boots up nice and after about 10 minutes its starts the BSODs.
2. while buying RAMs in the store I happen to see a cheap 20 euros cooling pad. (I actually bought it for the setting up viewing angle for long working hours). It has a 2 little fans that run by power from USB port of the laptop.
With a single 2GB kingston stick, win 7 64 bit, on the cooling pad, I ran 5 quicktime videos simultaneously AND started to work normally on the laptop, surfing, word documents.. etc. NO BDSODs for 8 hours straight !
(BTW I learnt that win7 64bit on 2GB works flawlessly ! Well you cant start 5 videos at same time though)
3. Well at this point, it could either have been the new RAM stick that was good ( and my old crucial stick was somehow bad.) or it was genuinely the heating based issue.
Next test, I turned on my laptop, with cooling pad off. Laptop booted up and worked for about 5 minutes, I moved the settings from powersaving to high performance, another 2 minutes and BSOD !
As a positive control, I connected the cooling pad, at high performance, same kingston RAM, no BSOD from last 1 hour !
and on touch, the laptop is MUCHH cooler.
4. I think, all along it was the heating issue which might have aggravated due to my recent India visit (40 degC !).
Tx2Z is infamous for its heating related issues (see here
Time to throw in the towel? You judge please.. )
So as long as i work on cooling pad no BSODs and as soon as i turn the fans off the cooling pad off, in about 15-30 min BSODs start.
Question: Is there any permanent option of attaching a cooling pad permanently to my laptop ? not sure if such a thing exists, or else a new laptop
Thanks a lot to everyone who gave suggestions.
cheers