RATIONALE: Drugs used in chemotherapy such as cyclophosphamide, methotrexate, and fluorouracil use different ways to stop tumor cells from dividing so they stop growing or die. Monoclonal antibodies, such as trastuzumab, can locate tumor cells and either kill them or deliver tumor-killing substances to them without harming normal cells. Combining chemotherapy with trastuzumab may kill more tumor cells.