The PRG thing is interesting. That could be a side-effect of the greater problem you are seeing (the excess errors being returned), and it could the root cause — if the PRG button were malfunctioning, that could also potentially cause some issues.
As a test, with the EggBot plugged into both power and USB, try pressing both the RST and PRG buttons at the same time. Then, release RST first and then release PRG. If both buttons are working correctly, that should put the EBB into a “bootloader” mode where it flashes red and green lights back and forth rhytmically. (Press RST again to exit this mode.)
Does that appear to work?