This randomized phase II trial is studying three different schedules of gemcitabine hydrochloride and tanespimycin to see how well they work in treating patients with stage IV pancreatic cancer. Drugs used in chemotherapy, such as gemcitabine hydrochloride and tanespimycin, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving more than one drug (combination chemotherapy) may kill more tumor cells