sounds like you have a finely balanced valve spring, which may be a little weak(try stretching it a little, cut spyder spring can also work) or try some of the items below. With regulated hpa you should be able to run down in the 500-700 psi range. Just remember that too low of a pressure will cause chuffing during ATing.
your may need to screw your main pump rod(the big one, not the one going to the bolt) in a little bit further into the pump handle. Just make sure you know where it is before you move it in case you need to put it back where it was. If it is entering the gun to far, you might be cause the valve to open slightly when pumping fast.
the adjustable hammer screw moves within the hammer. If you adjust it too far one way it is possible for it to lose contact with the threading and free spin without going anywhere. Try adjusting it both ways a bit to see if it will move back and forth.