RATIONALE: Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. Some tumors become resistant to chemotherapy drugs. Combining PSC 833 with chemotherapy may reduce resistance to the drug, and allow more tumor cells to be killed. It is not yet known whether combination chemotherapy plus PSC 833 is more effective than combination chemotherapy alone in treating patients with relapsed or refractory multiple myeloma.