Help! Stepper stalls high speed problem?

erm...stepper motors always try to run at full current.
Steppers aren't like most electric motors in that current is directly proportional to load, steppers take a constant current.

Some drives will reduce current if not moved for a set period, but the nature of steppers is they take a constant current

There are drives that can specifically change current on the fly, such as LAM drives but they are not common.

What causes the major drop off in torque with stepper motors, is back emf. Without enough voltage to ensure the back emf is overcome to drive the rated current through the motors windings, torque drops off.