Help! Stepper stalls high speed problem?

I wouldn't lower current, as then you're lowering torque (current = torque for most motors).

Most steppers can be run at temps too hot to touch, some even higher.
The key point to consider, is most heat comes from the rotor, so by the time the case has warmed up, the rotor is even hotter.
Get the rotor too hot, it'll demagnetise, and your motor is only really any good for the scrap bin at point.

My guide is that provided the motor isn't getting too hot to put your hand on within 5-10 minutes, you're probably safe.
If it's too hot to hold your hand on in under 5 minutes, then you're probably pushing too much current through it.
60-80degC after an hour or more running is perfectly acceptable, unless it's somewhere exposed and somebody getting burnt on it is a real risk, in which case you probably want to overspec the motor so it can be run cooler.

60-80 deg is not right. The motor should have a spec for the temperature insulation breaks down. Usually around 130 deg C
As long as you don't get that hot its fine. eg. So Andy's 105 deg is OK.
I might add this temp can be modelled as an increase over ambient but its complex.

One thing that is not well understood is the motor only requires full current when accellerating and decellerating as thats when its working. At other times, the power reuirement is low (maybe 20-30%)
This is where Lam Technology driver's boost function comes in. Linuxcnc can tell the driver when its at constant velocity and Lam can reduce the current accordingly to allow the drive to cool.

Anyway, we are are well of the track. 48V is the way forward. As Andy says you can reduce the current if it runs too hot and misses steps. It probably won't.
My motors run at 90 volts with Lam Drivers and boost.

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.