I've searched the forums and haven't been able to find anything that really fits this question. If it's covered somewhere else, I apologize in advance...anyway, here it goes:

I recently bought my 2004 G35 sedan. It has 74K miles and it's in great condition. I had only rattling issues with it, or so I thought until I drove it for a longer period of time..

After that somewhat random interval of time the PS fluid level drops significantly, but never emptying the reservoir. I only noticed since I got the god awful whine one day after running it for over an hour or so. What I found odd is that the reservoir refills once the car is turned off and cools.

I immediately thought "Air!!" so I bled the hell out of it and while I have gotten some air out, it doesn't seem to help clear the issue.
I believe I may have a leak on the suction side that lets air into the system, but it only happens after reaching a certain temp. There's really no froth or bubbles in the reservoir when I check though, so I'm having trouble really convincing myself of that as well.

Has anyone else run across an issue like this? If so, do you have any points of interest to check, or advice as to what the ultimate culprit might be?

Thanks in advance! It's driving me nuts and is the only thing keeping me from taking my new (to me) G on a nice long highway run.