Same problem with mine after the 4.0.4 update.  I called AT&T tech support and they wouldn't admit there was a software problem.  They suggested I do a factory reset on my phone.  So, I backed up to Kies and made sure all my contacts were in my Gmail.  Reset phone.  I had a lot of apps that didn't come back over and a lot I didn't want to use anymore.  Since doing the restore I have only had one shut off in two days.  Before it was doing it 3 or 4 times a day.  Battery llife seems to be better too so far.  I'm pretty sure if I hollered more they would let me do an upgrade since I am due in March.  I'd really rather just have my S2 work like its suppose to.  So far its much better so we will see.