Hi...have also had this problem with the power steering switching off and on, and was diagnosed to be a faulty ECU. We replaced this with a second hand unit which cleared the problem.....momentarily. \Bear in mind that the soldered joints are suspect,and if you are ok with an iron, check them all at the same time.
Next, and I think really important, have a diagnostic check of the various sensor operate voltages,as I'm suggesting that if your battery is even slightly suspect, replace it, or give it a decent charge. this happened to me just last month( the two error codes in the ECU hinted that the steering sensors on the column had not operated). After a decent battery charge, and reconnecting into the car,no problems with steering. Until the error counter in the ECU is reset, any codes will remain held there.
Ross, September 2012