Couple of thoughts.

First - 48V and NEMA34 motors don't really go together. The high-inductance motors need a fair bit of volts to get them going. It's possible that things have tightened up a bit or moved slightly out of aligment since first building the machine, so more friction, so more likelihood of stalling? Does it still happen if you reduce acceleration and max speed? Try low values just for testing purposes.

Second - and possibly linked to first. Sounds like you are using a switch-mode power supply. These are really not ideal for stepper motors. Steppers need high pulse currents even if the average is low, and it's possible that these peak currents (possibly increased because of a bit more friction as above) are tripping the over-current limits on the PSU. Ideally, you should be looking at about 68V from a linear power supply - details and how to build are available on this site!