Broken DR - never encountered this one

one more bug inside the scene"

when i firs start the render, the LC computes from 596 hrs…down and takes so long…like 2 hrs to compute
after thati cancel the render and restart it and all of the sudden the LC takes only 7 mins!!!

Whats going on?

I am in the middle of a really tight deadline, need to render over the weekend some very hi ress images …PLEASE HELP!

PS:
installed the freshest vray and its stil broken :-/