Hey im having some trouble here,

Today when i went to work everything was fine with my z(2006). it started to rain a few min before i got off but I got in my car to go home and i noticed the steering wheel was really stiff.I remember driving an old beat up minivan it it kinda felt the same and when i took it to the mechanic he said the power steering pump went out.

So i managed to get home(not a long drive) but before i got home i stopped the car and turned it off, waited a few minutes and then started my z up again and the powersteering worked. I went to the gas station to put air in my tires cuz they were kinda low and when i started the car up the power steering wasnt working again, it was very stiff and i drove around a bit then the power steering started working again..and then it stopped again..and finally it came back again...im like WTF!

So any ideas? I just checked the power steering fluid resevoir and its ok..