modem hangup

[Mike McClain <mike.junk@nethere.com>]

Hi,
    Running Debian 7.3 on an Intel P3 D815EFV 1GHz with 512M ram.
My modem, a US Robotics 5610C, connects to the ISP via ppp and keeps
hanging up with out my intervention.
    This happens most often at 2.4 minutes -.1/+.3 min. after connecting
and other times seem random, yet I can stay online all night when
doing a distupgrade.
    A google search led me to reduce the mtu from 1500 to 1200 to no
avail. I've got pppd keepalive running and may disconnect immediately
after receiving an LCP EchoReply with the logs only showing 'modem
hangup' and no suggestion why or by whom.
    Any suggestions on how to debug this problem would be appreciated.
Thanks,
Mike

--
"The only way that problems get solved in real life is with a lot of
hard work on getting the details right."
    - Linus Torvalds