Trouble connecting through NAT-Router

I'm facing a strange problem for some time now and still don't know a
fix. The trouble is with clients behind a NAT-Router. I connect with MS
Access 2000 using the 07.03.0200 ODBC driver, PG is running on linux in
version 7.3 and 8.0.1. Now this works fine for an unspecifiable amount
of time (sometimes 2 seconds, sometimes 8 hours). Suddenly the
connection to the database server breaks up - the forms in Access show
#name?, every query returns an exception.
On a really old router I found an option called "TCP Masquerading
Timeout" and playing with it seemed to have an influence on the
phenomenon.

My assumption now is: if the tcp-connection breaks up because the router
doesn't have enough memory for all the connections, the driver does not
reconnect to the database server. Actually this seems quite unrealistic
to me, but what is actually going on?

Thanks for help,
Tom