It sounds to me like something on the SQL Server is closing the connection after a certain amount of time. Might want to check with your SQL DBAs to seem if there is some security setting/policy doing that.
The first thing I would check is whether or not you have a continuous connection to the server, even short interruptions of a few seconds can force access to croak.
However, this message looks more like your server is terminating the connection, as in your server administrators only allow continuous connections of a certain period before timing out. If the tables are related to 3rd party software they may also have some internal controls that are enacted after continuous connection times are exceeded.
I am having the same problem with connection problem with SQL. I found out that there's something wrong with my DBA connector and now its smooth as ever.