start dip with -v for verbosity and tail /var/log/messages
i'm running 1.3.85 with cslip without troubles, except for the usual
line noise. I can usually only get 2/3's the performace capability
of my modem.
I hope these people reporting poor PPP performace are doing the
equivalent of the USR's ATI6 command, which shows line quality
impairments. Line quality can vary from call to call.