Thanks for your replies.

Yes, live view with AF-S does not seem to find or hold focus .. i tried it again tonight

** contacts are clean ... camera is only a few weeks old and primary test lens is a Sigma 50MM prime F/1.4

It is weird, because in viewfinder mode, the body/lens combo does take nice shots.
I have tunes my 3 lenses using "Focal" and that did make quite a difference to sharpness.

My concerns about returning camera for correction is

* I am in Australia (Melbourne) and i have no idea if the local NIKON service centres have much experience (or the correct equipment) to service the D800. I don't want them to fix it and make it worse.

* The camera originally came from HK (bought online) and came with a European warantee... so not sure how claiming a warantee fix will go.