Unexpected Shut down when moving macbook pro on my 'lap'

In the last month or so, I've been suffering from an unexpected shut down of my mac book pro - which is quite annoying.

The issue seems to be (and it is reproducable) that after a few minutes (or seconds sometimes) of moving my mac from a flat surface (e.g. a coffee table) onto my 'lap' the computer just shuts down (no warning).

I can reproduce this by holding my mac in both hands above my lap and rotating away from me (i.e. screen closer and keyboard futher away)....

After some googling, I think this could be related to the sudden movement sensor (and a suggestion was to reset my smc by doing shift-ctrl-option-powerbutton when the mac was powered down). But this hasn't worked.

Anyone experienced similar issues?

I'm using a 17inch early 2011 mac book pro, running OS X 10.8.3 with the latest software updates installed.

It doesn't seem to matter what I'm doing, i.e. watching videos, writing emails, using lightroom, photoshop.

I wonder if this is related to any recent udpdate?

Thanks

Ioant