Problem streaming from Mac Mini with Drobo

I wonder if anyone could help with a problem that I am having with my new ATV ?

I have just upgraded to the new ATV from one of the original 40g ones and now I'm having a problem with streaming. I'm streaming from a mac mini which I use as a media center running Plex and itunes with a Drobo attached. When I start up the ATV and start streaming a movie from iTunes on the mini it's fine for about 10mins then the mini and drobo sleep. I have the sleep setting set to 1 hour but still it sleeps after 10-15 mins, this never happened with the old ATV, Im at a loss and would be grateful of any help.

Thanks in advance