Sounds like the pump might be wearing. OE or aftermarket? When the problem occurs, can you solve it by revving the engine slightly?
What fluid are you using? FWIW I had similar issues when trying to quickly turn the wheel at idle with a hot engine. I'd max the flow rate of the pump and could only turn it so fast. Switched it to RP fluid (little thicker than the redline that was in there before) and it seemed to alleviate it. Take that how you will. I'll be replacing my pump at some undetermined point in the nearish future. Depends when I can be bothered with it.
p.s.(heh, get it?) use an OE pump. When I worked in a shop, I lost track of how many come backs we got from folks complaining of the same, only to find that pretty much every aftermarket power steering pump didn't work worth a damn. Had a chat with parts, switched OE only and haven't had an issue since