I assume you have ruled out the possibility of a bad cable?  bad chip?
The latter would be unlikely.

Try turning off the getty on the port and just cat'ing stuff to the port.
The default setting should be 9600 baud, no parity.  You can experiment
with various settings by:

	(sleep 10000 < /dev/tty0) &	# hold open the port
	stty whatever < /dev/tty0	# change things
	cat /etc/motd > /dev/tty0	# see what happens

The stty line may need to be ">" depending on whether your stty uses stdin
or stdout.

P.S. a 370 does have an unbuffered UART.  I have one at home (where I use
the dca for a SLIP connection at 19.2k) and one at work (where the dca is
the console line at 9600 run into another machine).  I don't recall seeing
this problem.

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