Maybe this is a myth, but I believe on wowwiki or wowhead they say it might be that he decides to spray if the tank moves out of melee range. We were noticing a lot more random sprays last night, and then after someone mentioned the above I was careful to not be moving while he turned and injected people. It seemed like this way he was always only spraying me and not the raid.
Maybe it was luck, but I think I'll be sticking to that in the future.
Also, it's annoying to have a DK in the group (unholy I think?). They have some kind of plague aura or strike thing that looks very similar to Grob's poison clouds. Confused me sometimes if I wasn't watching for the green flash close enough.
I was also having issues with him wanting to move directly on top of me, then having to turn around to reposition himself.

