Q: power steering not working

I've been having problems with misfires on my cylinders – 3 different ones. After taking it to the repair garage I only had 1 misfire in my first cylinder. Now my power steering seems to be failing. While I am driving I feel a tug on my steering wheel. I drive OK then all of a sudden my power steering stops functioning causing my steering wheel to lock up and forcing the wheel into the direction needed. I would have to tug at my wheel - left and right – to stay in control. This has almost caused an accident! I got a diagnostic check, but nothing came up regarding the power steering. They flushed out my old fluid and replaced it with new fluid. I still have the same problem. Help please. Thanks.