SQL ODBC Error when idle

Hi
Got an odd one on an application, I'm using Access 97 to talk to SQL
Server (also we have 2002 and 2002 versions of the same application as
the client has many differant PC builds!!)
Just done the SQL conversion, all works well but if I leave the screen
open for more than about 30mins I get an "ODBC Call Failed" error pop
up when the form gets focus back, its as if the connection has timed
out. The last record is still there but has been saved, its as if the
connection has timed out after a period of inactivity???
Any ideas? The application recovers as it reconnects itself but before
that all the bound fields have the word ?#Error in them as of course
the bound record has gone
Cheers
shaun