This is an old problem with no easy solution as of now. For a better description of what is going on here, you can read <a href="http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RPAGEID=137&HOID=506500000005000000D3370100&HTHREAD=000079827&UCATEGORY_0=_49_%24_6_&UCATEGORY_S=0";>this discussion</a> where I answered a similar question a while ago.
Ed
