I'm in the same boat, DS goes through these stages that will last a few weeks then he'll magically come good again - think it might have something to do with the full moon at the moment! Time out doesn't work for us anymore, we'd be sitting there doing a puzzle or something and DS would look at me and say "I want to go to time out Mummy" like it was some kind of novelty!! So we've canned the time out corner and put him in he's room, away from everyone this seems to work most of the time. I usually get down to hes level, look him in the eye and talk to him about the situation but lately he won't even look me in the eye. The other day while trying to get him to hold my hand while we cross the road he screamed "leave me alone!" and burst into tears and we ended up standing on the side of the road (with a heap of on lookers) while I tried to explain that he has to hold Mummy's hand etc etc- nothing worked so I ended up walking back to the car pushing a pram and carrying a screaming three year old!! I don't know but it's like he's tired all the time lately but yet if I put him to sleep during the day, even for 1/2 hr he won't go to bed until really late that night (we put him to bed but he'll keep getting up every 5 minutes using the excuse that he needs to go to the toilet)
I've just got hold of a copy of the mighty toddler from a friend, I haven't started reading it yet but i'm hoping it will have some helpful hints in there!