Mustang Power Steering Failure

[MustangLVR]

The other day when I was trying to park my 2008 Mustang GT, the steering when solid as though the power steering had completely turned off/failed. However once I started moving again above 5mph It all came back. It seems to be an intermittent problem. Has anyone had this sort of problem before if so how was it rectified? It also made a groaning sound when I did actually move it.. It doesnt happen all the time just some of the time.

p.s. I have checked the power steering fluid level and that all seems fine.

regards Alex