Motor runs fine up to 4400revs but halts at 4500

Hi,

I've been struggling to get this motor to run but I thought I had it nailed down until I ran it loaded.  Using the GUI (loaded with Lab10a) I can increase the speed by 500 RPMs up until I get to 4500 revs.  At this point the motor abruptly halts but the current still flows about 4.5A.

Here is a screen grab of the GUI:

Is there something that is obvious that I've gotten wrong?

Thanks,

Richard