General view seems to be that motor case temp of about 100C is about the limit, and that's based on manufacturer recommendation that wire insulation temp should not exceed 130C and there's about 30C difference between wire and case temps. Manufacturers do not seem worried about magnetism. My own steppers run at about 60-65C (based on the "just to hot to keep hand on" test) and that seems to be pretty normal.

I have two motors driving X and the only stalling has been when I've had too-rapid "rapids" configured - max X speed is at about the corner speed for my steppers so they are starting to lose torque and tend to stall. The driver concerned tripped with a stall fault. Low-speed stall is clearly something else. What kind of moving mass are you talking about, and what acceleration settings? Odd that it only happens after running happily for a while, though.