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Network problem

Setup:  All systems running Redhat 5.2, kernel 2.0.36-3 (custom
compiled), and latest rpm updates.  All on 10baseT plugged into a hub. 
Crossover cable from ISDN Ascend pipeline (employer provided) to the
uplink port on the hub.  ISDN is the 10.x.x.x network.

- A 386 with 2 ethernet cards (one on a 10.x.x.x address connected to
ISDN, other on 192.x.x.x address--my internal network).  I'm trying to
learn about routing/firewalls though not everything is working yet.  At
this point, only the 386 can ping machines on 10.x.x.x and 192.x.x.x
network.  3c509 & e2100 are compiled directly into kernel (not modules)
- A laptop with 3com pcmcia card (also on 192.x.x.x) (3c589 is kernel
- A P5-233 with 3c509 card (also on 192.x.x.x)  3c509 compiled directly
into kernel (not a module)

Problem:  At times the network just dies.  Doesn't seem to matter if the
machines have been up for a long time (386 has been up for 14 days so
far) or just recently booted.  Problem appears on all 3 machines, ie, a
simple ping to another machine returns nothing.  Telnet, rlogin, ftp
etc, also don't work.  I don't think it's hardware because everything
seemed to work just fine in the past (same hardware).  The basic
difference is the 2 ethernet cards in the 386.  It doesn't seem to make
any difference if the ethernet setup is compiled as modules or not. 
Problem still exists.

I ping machine A from machine B and get no response, (pinging machine B
from machine A also gives no response).  Makes no difference what
machine A or B are, it happens on all 3 I have on the network.  If I
have both machine A and B pinging each other at the same time, after
about 5 minutes or so both pings start showning activity.

The strange part is that I have syslog logging icmp info (pings).  The
initial ping from machine A to B shows up in machine B's syslog but
returns no response on machine A's console.  And vice versa, so I know
network traffic is getting through right away otherwise syslog wouldn't
be showing this immediately.  The lights on the hub also show immediate
activity.  I just don't understand why the network disappears suddenly
and then starts working again later.

Any ideas?
Scot W. Jenkins