97LT

Hey I am new to this website and wanted to start my first forum. Sorry if there is already one that answers this question/problem but I couldnt find anything on it. I just bought a 97 z28 and my power steering works fine as long as my foot is in the throttle. Once the RPM's get down to idle its like the power steering completely goes away. Fluid looks fine. I'm not a mechanic of any kind and dont know what to look for? New pump? Rack and pinion? Any feedback would be great from someone that has had this issue before. Thanks.