The usual answer, from back in the day, was to make sure hardware flow
control was on and working at higher speeds. If you don't have that,
you are going to lose > 9600 baud. I spent way too much time pulling
fat cable to enable this after we'd wired the building with 3-wire
cable for the VT52's and cute 'headphone' plug-in jacks back in my
high school days...
Generally, DEC didn't have hardware flow control at that time because it was
not in the standard. I'm pretty sure there was no hardware flow control
available on the VT100. It used XON/XOFF.
Regards,
Peter Coghlan.