Overheating

I have a 1997 Lincoln Continental that has 119,000 miles. It is overheating for no obvious reason. I have taken it to a shop who has driven it all day long (long enough to use a whole tank of gas) and never overheated once. But as soon as I got it back home there was coolant pouring out of the coolant resivour and leaking everywhere. I replaced the radiator cap thinking it was that, but it is still doing it. It passed a pressure test, and the belt is fine. I can tell though that the power steering pump is going out. There are times when I could not turn the wheel at all, and others when it turns just fine. Any help would be great.

Thanks