[c-nsp] Telnet session dropped

[Roy]

I use RANCID ton a number of routers.  About five days ago, it started 
failing on three routers.  If I manually connect to these routers, it 
seems to work for a minute or so and then the telnet session gets 
disconnected.  The disconnect only occurs during a data transfer such as 
"show conf"

I tested it from several places so the telnet client doesn't seem to be 
a factor.  The routers are all on different ISPs and in different 
cities.  The only common denominator between the failing routers is they 
are all set to use a port other than 23.  This is done by natting the 
external port to port 23 on a null interface.

Rebooting the routers didn't help

Any ideas?