10K,
I may have missed it it your posts, but what are the specs of your lead screws?  Perhaps you are asking for too much rpms trying to get faster ipms with a screw pitch that is not capable of the speeds you seek.
Regards,
John Champlain

10K, in this post you will find a link to a video of a very nice machine running at roughly 758 IPM.

http://www.machsupport.com/forum/index.php/topic,22495.msg170685.html#msg170685

Brett

Screw pitch on X and Y is 5 turns per inch.  Steppers are 2000 pulses per rev, so 5 x 2000 = 10,000 pulses per inch.

As I mentioned, I can get the X axis to at least 200ipm with no lost steps if it's running by itself.  The problem comes in when X and Y are running simultaneously, as happens for circles. 

Going from a 4A to a 15A power supply does not make any difference.

10K,

Sounds like your steppers may be limiting higher, more reliable, rpms past 1000.

John Champlain