Are you using XML as a data source?
I've had similar problems. I usually end up doing running totals for this but calculating on change of a unique record identifying field. Try setting it up with Field_A as the Field to Summarize, Type of Summary=Sum, Evaluate On Change of field=Field_UniqueRecordIdentifyer, and Reset as needed.
I've often had programmers create a new field for this purpose that is just a number field that increments by one for every new record. That way each record will have a unique number with no duplicates and you should get your total as needed.
Hope that works for you, or at least gives you an idea for something that will.