LIGHTS FADING over time

Hi I have got another problem on top of the jitter on my renders.

Still havent worked that one out why yet..! Il let you know if I do.

It seams the light dims over the render. I have attached 2 images. The start and endframe of my render.

I am suing 3d studio max 2010 64 bit, with vray 1.5 sp3

This is lit up by a hdr, but I am having same issues with a normal vraylight aas well.

Cheers

Thanks for your time and help.