Power Steering Issue

[BluRaySRT]

I have a '71 Cuda with a stock hemi and stock exhaust (no headers).   I also added stock power steering a couple of years ago.   The problem I'm now experiencing only occurs after the car has been driven more than a half hour with outside temp above 60 degrees.   The symptom, as I can best describe it is that the power steering begins to 'chatter' when making even the slightest turn. It is terrible when you are trying to park as it is both annoying and noisy.  As you turn the wheel you can feel it resist and then release repeatedly causing what I am calling 'chatter'.   Because it seems to be related to heat,  I have changed the PS fluid to high temp fluid and I have also added an external cooler that is placed in front of the lower part of the radiator.  (All Hemi's came with a PS cooler but it was small and located near the exhaust manifold. I don't think they did much, but the engineers must have thought them necessary. I don't have one as they are now impossible to find, so I found an aftermarket one.)   I would like to better diagnose this problem before I start replacing parts.  Thanks in advance for your advise and suggestions.   

[Cuda_mark]

Are you using the original HEMI federal pump? Just curious.

[BluRaySRT]

Yes...  it's the Federal one.   

[Mr Cuda]

 Are there any chances of a mismatch on the pulleys or belt?  Wide 3/8 to  5/16, early-late.
A belt not riding high in the pulley V will make that noise when the belt is warm under load. The bottom of the belt should not touch the center bottom of the V.
 Proper belt fit will have the belt top at or above the pulley side wall V.
 

[7212Mopar]

Overheating issue. The pump is constantly spinning turning the fluid over when driving but you are not turning much. Energy put in that is not used turns into heat. Try new power steering fluid with higher temp rating might help or install a PS cooler.

[BluRaySRT]

Yes,  I believe that it may be an overheating issue, but I have already installed a cooler and changed to a high temp fluid. 

Now I'm thinking that the pump might be defective.  I guess I could just change it out.  I don't know how else to diagnose it.

[Ramman]

I'd get it to the point it's making the noise. Shut it off and stick my candy thermometer in the fluid reservoir. What temp. is the fluid? Did the cooler and fluid change have any effect? If not, i'd say it's pump time.