I am able to successfully connect to Internet Navigator
(http://www.inav.net), but after a period of time, often less than a
minute, performance drops to unacceptable levels.  I have connected to
Internet Navigator using previous versions of Red Hat and I have not
experienced these difficulties.  I have incorportated the suggested fix in
http://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=7180
but it did not help.

Here is a transcript of my /var/log/messages file, with username and
password elided.
Nov 26 23:33:14 localhost ifup-ppp: pppd started for ppp0 on /dev/ttyS1 at
115200
Nov 26 23:33:14 localhost modprobe: can't locate module char-major-108
Nov 26 23:33:14 localhost pppd[11834]: pppd 2.3.10 started by root, uid 0
Nov 26 23:33:15 localhost chat[11843]: abort on (BUSY)
Nov 26 23:33:15 localhost chat[11843]: abort on (ERROR)
Nov 26 23:33:15 localhost chat[11843]: abort on (NO CARRIER)
Nov 26 23:33:15 localhost chat[11843]: abort on (NO DIALTONE)
Nov 26 23:33:15 localhost chat[11843]: abort on (Invalid Login)
Nov 26 23:33:15 localhost chat[11843]: abort on (Login incorrect)
Nov 26 23:33:15 localhost chat[11843]: send (ATZ^M)
Nov 26 23:33:15 localhost chat[11843]: expect (OK)
Nov 26 23:33:16 localhost chat[11843]: ATZ^M^M
Nov 26 23:33:16 localhost chat[11843]: OK
Nov 26 23:33:16 localhost chat[11843]:  -- got it
Nov 26 23:33:16 localhost chat[11843]: send (ATDT6268888^M)
Nov 26 23:33:16 localhost chat[11843]: expect (CONNECT)
Nov 26 23:33:16 localhost chat[11843]: ^M
Nov 26 23:33:48 localhost chat[11843]: ATDT6268888^M^M
Nov 26 23:33:48 localhost chat[11843]: CONNECT
Nov 26 23:33:48 localhost chat[11843]:  -- got it
Nov 26 23:33:48 localhost chat[11843]: send (^M)
Nov 26 23:33:48 localhost chat[11843]: expect (ogin:)
Nov 26 23:33:48 localhost chat[11843]:  38666 V42bis^M
Nov 26 23:33:50 localhost chat[11843]: ^M
Nov 26 23:33:50 localhost chat[11843]: ^M
Nov 26 23:33:50 localhost chat[11843]: login:
Nov 26 23:33:50 localhost chat[11843]:  -- got it
Nov 26 23:33:50 localhost chat[11843]: send (pomije^M)
Nov 26 23:33:50 localhost chat[11843]: expect (assword:)
Nov 26 23:33:50 localhost chat[11843]:  ^M
Nov 26 23:33:50 localhost chat[11843]: login: ******^M
Nov 26 23:33:50 localhost chat[11843]: Password:
Nov 26 23:33:50 localhost chat[11843]:  -- got it
Nov 26 23:33:50 localhost chat[11843]: send (********^M)
Nov 26 23:33:51 localhost pppd[11834]: Serial connection established.
Nov 26 23:33:51 localhost pppd[11834]: Using interface ppp0
Nov 26 23:33:51 localhost pppd[11834]: Connect: ppp0 <--> /dev/ttyS1
Nov 26 23:33:52 localhost pppd[11834]: local  IP address 205.160.208.221
Nov 26 23:33:52 localhost pppd[11834]: remote IP address 205.160.208.248
Nov 26 23:33:52 localhost pppd[11834]: primary   DNS address
199.120.107.101
Nov 26 23:33:52 localhost pppd[11834]: secondary DNS address
199.120.107.102
Nov 26 23:34:03 localhost gnome-name-server[11924]: starting
Nov 26 23:34:03 localhost gnome-name-server[11924]: name server starting
Nov 26 23:35:40 localhost kernel: hdc: lost interrupt
Nov 26 23:35:50 localhost kernel: hdc: lost interrupt
Nov 26 23:35:56 localhost gnome-name-server[11924]: input condition is:
0x10, exiting
Nov 26 23:36:00 localhost kernel: hdc: lost interrupt
Nov 26 23:36:30 localhost last message repeated 3 times
Nov 26 23:39:41 localhost gnome-name-server[12022]: starting
Nov 26 23:39:41 localhost gnome-name-server[12022]: name server starting
Nov 26 23:41:05 localhost kernel: hdc: lost interrupt
Nov 26 23:41:45 localhost last message repeated 4 times
Nov 26 23:41:49 localhost gnome-name-server[12022]: input condition is:
0x10, exiting
Nov 26 23:41:55 localhost kernel: hdc: lost interrupt
Nov 26 23:42:15 localhost last message repeated 2 times

I think that I start getting messages like
Nov 26 23:41:45 localhost last message repeated 4 times
when the slowdowns occur.

Here is my ifcfp-ppp0 file
DEVICE="ppp0"
ONBOOT="no"
USERCTL="no"
MODEMPORT="/dev/ttyS1"
LINESPEED="115200"
PERSIST="yes"
DEFABORT="yes"
DEBUG="yes"
INITSTRING="ATZ"
DEFROUTE="yes"
HARDFLOWCTL="yes"
ESCAPECHARS="no"
PPPOPTIONS=""
PAPNAME=""
REMIP=""
NETMASK=""
IPADDR=""
MRU=""
MTU=""
DISCONNECTTIMEOUT=""
RETRYTIMEOUT="5"
BOOTPROTO="none"
PPPOPTIONS="noccp"