Randomly failing SQL Server 2008R2 used by Access 2010 front-end via ODBC
As Max says it sounds similar to a problem we had under a similar situation, especially for those working remote to the main company network.

Ensure that under the Access database Options Advanced tab that the OLE/DDE timeout is set to something like 300 and not the default of 30.

Also in any VBA requests to the database adjust the ADODB command.CommandTimeout value to a high value of at least the same 300, this will stop early timeouts and lost connections. Since we did this we have had no dropouts, it only uses the extra time as needed so do not worry about system locks from this.