stepper motor losing streps at limited current
If you lower the current, you lower the torque. If the torque is insufficient for the load, you miss steps.

As you increase the speed, two things happen:


The back EMF from the motor increases. A constant current driver will raise the motor terminal voltage to attempt to compensate, but at some point the supply voltage isn't high enough, so the driver can't increase the voltage any more. Thus, motor current decreases.
For most mechanical loads, higher speed requires higher torque. If you are getting to that higher speed in the same amount of time, that's higher acceleration, which requires more force. Some loads also have friction which increases with speed. Think of your car.


You could do any of these things to attempt to remedy the problem:


Don't decrease current, using a larger motor which can more effectively dissipate heat if necessary.
Reduce the mechanical load, by making it lighter, or by providing mechanical advantage (which will also reduce speed).
If you having the problem #1 above, increase the supply voltage, making sure it remains within specifications for your driver and motor.