This is a commonly occurring issue using CGMiner 4.9.0 and the Antminer U3. Sadly I could only get my Antminer U3's to run well by using CGMiner 4.9.0. So it's either a very persistent problem with the Antminer U3's in general or it's a software issue.
You can check out the main Antminer U3 support thread to read about how most people's units cut out every 24 hours like clockwork. Not that it helps, but maybe someone will eventually figure out a fix. I know I couldn't...
I cut the cord and put it on a 30 amp psu, hashing fine now, been a few days so fare. I have been using CGminer 4.9.0 from the start.