TDBChart: need help

Hi & thanks for reading...(also posted in cppbuilder.component.using)

I need a bit of help here.  I am attempting to use the code listed below to
open different *.dbf files.  Obviously, all files need to have the TimeStmp
field based on the SQL WHERE clause.  But the other fields can change in
name and number.  Any file will at work first pass and appears in the
TDBChart...suppose the first file has these fields afield,bfield, and
TimeStmp.  Now I wish to open a second file with fields
bfield,cfield,dfield, and TimeStmp.  I get a runtime exception indicating
that "QryFiltered: Field a not found."  The exception occurs on the query
Open command (see below in code).  I have run test code when the query is
not hooked via the TChartSeries->DataSource property and no error and
obviously no graph!  IOW, I keep all code the same but never set the
DataSource property (see code below).

I guess the real question is how can I alter the underlying datasource (a
SQL query using TQuery) of a TDBChart at runtime.  I can change the query no
prob...but what do I need to do to "prepare" the TDBChart for this change
(disable, clear series, NULL datasource, ...WHAT????????

Thanks,
Rich

Other Info:
BCB5 Enterprise WinNT & Win98
QryFiltered = TQuery
DBChart = TDBChart

--------- CODE
BELOW ----------------------------------------------------------------------
------------------------------------
QryFiltered->Close();
QryFiltered->SQL->Clear();
QryFiltered->SQL->Add("SELECT * FROM '" + OpenDialog->FileName + "' WHERE
            TimeStmp BETWEEN '" +      BeginDate->DateTimeString() + "' AND
'" +
            EndDate->DateTimeString() + "' ORDER BY TimeStmp ASC");
QryFiltered->Prepare();

**** ERROR OCCURS ON THIS LINE SECOND PASS THROUGH ****
QryFiltered->Open();

for(int i=0;i<Num;++i){
    ****IF COMMENT OUT NEXT LINE CODE DOES NOT THROUGH EXCEPTION ****
    DBChart->Series[i]->DataSource = QryFiltered;
    ...set yvalues, xvalues, scaling yada yada yada... etc.
    }