So you are doing protocol translation,
allowing PPP users to access the X25
server?
You might give the latest 12.2 (or 12.1) a try;
if you still have problems, please collect
"sh user", "sh caller", "sh translate",
"sh process | include Virtual", and
"sh stack #", where #! is the process
(PID) number from the output of "sh
process".