Ok, when your motors stall it means from my experience that you are trying to run them too fast. If I take mine over 700rpm then I get the same problem. One solution for this (but not tried it) is more voltage, what voltage power supply are you using??

As for your feed rates, you should be able to run as fast as your rapid rate. So you say 20 inch's a min (why are you not working in mm??) is your rapid rate so if you put a feed rate in ie G01 X100 Y100 F500 (this is mm min) then this should give you feed rates similar to your rapid speeds.

If you want more speed then you could either drive the screw through a step down pulley system which will reduce your torque but increase your speed or put a larger pitch screw in.

Hope this helps...