As others have said, your speed is restricted by the kernel speed. I'm running 20,000 steps and the same total screw lead as you. With 25k and I max out at 75IPM, which is consistent with your results.
However, the stalling is a differnet problem. First off, you don't have enough voltage in your supply. Voltage should be many times the motors rating. I know it seems odd, but s that's the way it works. Probably why your motors don't get warm. Do some homework on steppers.
www.geckodrives.com has a lot of good info and calcs. You can determine if your small steppers are up to the task. This is a somewhat complicated topic which requires some study (I'm still learning).
You need to look at the acceleration. If you have it set too high, the motors will stall trying to get the mass moving, which is especially demanding during a direction reversal. Raising the speed increases this problem.
Incidentally, your computer should be way more than enough for Mach 3. Currently I am using an old Intel P3 850mghz. It is a dual processor machine, but the CPU runing Mach never goes above 25% CPU utilization. On the other hand I was using a 1.2Ghz laptop wiuth a VIA processor and it was often pegged, so obviously processor brand type does have a lot to do with it.
You might want to confirm you are using the correct pulse width.