Line speed on 3COM USR 56Kbps modem

[Benoy]

Hi friends

My  USR 56 KBPS modem is not able to connect to internet more than 33.6
kbps line speed
some time it is connecting more than 37 or above
then some junk characters are getting,  no username prompt

I am using RH 5.2 and KPPP for connecting (PCQ March)

modem speed is set to 38 kbps.

Please help

Benoy