nvision 3 tier hangs when performing drilldown

We are using a 3-tier install of nvision to run reports and perfrom drilldowns.  We are experiencing an issue with the drilldowns if another drilldown is attempted

after some period of time has passed.  For example we are able to select a cell and perform a drilldowns successfully, and repeat this process for other report cells soon thereafter.  When we attempt to perform another drilldown after some period of time has passed on the same report,  the cell is 'frozen'. - we are not able to select a new cell.   This usually happens  after  some period of time passes(15 minutes/1/2 hour).  At that point we have to use task manager to kill nvision and excel, and sometime a reboot of the workstation is required.  Is there some sort