fragmented packets on LAN

Does anyone know if packet fragmentation on your iSeries network can cause
appearance/manifestation of the problem of unexplained random dropped 5250
sessions?
Symptom: My 5250 interactive session is left active as if nothing happened
(I am sure you have experienced this) the 5250 client gets a new signon
screen. Other concurrent 5250 sessions between same source and destination
IP's are still running fine.

I have tried several 5250 clients, more than one iSeries and release level
and keep alive adjustments.

I have read through the midrange archives, reading most suggestions and
individual solutions/fixes.

As a test, I lowered a PC 5250 client NIC MTU to a level where packets are
no longer fragmented from the effected iSeries network.
The problem seems to be masked now.

This problem does not seem to happen on other iSeries networks I have
recently telneted to, which means my issue is local to an iSeries network
LAN path to the WAN, and not me, a particular iSeries or the WAN/internet.

With that said, does anyone have a suggestion on
1) Is this is a correct determination/likely cause for random 5250 dropped
sessions

2) what are the typical reasons for high packet fragmentation on a LAN?
old/failing piece of hardware? ISP? something configured wrong? bad nic? bad
wiring?

As always, THANKS!

Mark Villa in Summerville SC