Dynamic Query problem

Dear All,
I am new in postgres there for i have some problems. I am executing some type of Business rule in strored procedure using language plpgsql. When I am executing a Dynamic Query then It return an error which is some how unable to understand Please Help me It is urgent . I am attaching my code as well as error which appears to me.
I have noted that if table has same type of column then no error comes .

I am using RedHat 7.1 and postgres 7.1.2
Thanks

Regards
Dinesh Parikh
NSTL, New Delhi.

Function Code is described as:
drop function testfunc();
Create function testfunc()
returns int4 as '
Declare
Query text;
MyRec Record;
DBColName Text;
DBOut Text;
DBCount int4 := 1;
Begin
DBColName := ''X_1'';
For i In 1 ..10
Loop
If (DBCount =1 ) Then
DBColName := ''X_2'';
DBCount :=2;
Else
DBColName := ''X_1'';
DBCount := 1;
End If;
Query := '' Select ''|| DBColName || '' As Field '' || '' From TestTable '';
For MyRec In Execute Query
Loop
Raise Notice '' MyRec.Field = %'',MyRec.Field;
DBOut := MyRec.Field;
Raise Notice ''DBOut = %'',DBOut;
End Loop
End Loop;
Return null;
end;
'language 'plpgsql';

I have a table TestTable(X_1 int4, X_2 Int8);

Error Appeared as:

Jurassik=# select testfunc();
NOTICE: MyRec.Field = 211
NOTICE: DBOut = 211
NOTICE: MyRec.Field = 221
NOTICE: DBOut = 221
NOTICE: MyRec.Field = 231
NOTICE: DBOut = 231
NOTICE: MyRec.Field = 241
NOTICE: DBOut = 241
NOTICE: MyRec.Field = 21
ERROR: type of myrec.field doesn't match that when preparing the plan