Medical researchers looking into a puzzling outbreak of brain tumors at an Amoco Corp Research Centre say they have made an important step towards finding the cause of the problem. Since 1989, 17 cases of head tumors have turned up among research and development workers at Amoco's Naperville Research Center outside Chicago. Seven of the victims have been diagnosed with the deadly form of brain tumor called Giloma, all of them men, even though more than a third of the employees in the complex are women. Four of the glioma patients have died. Researchers from the University of Alabama at Birmingham and Johns Hopkins University in Baltimore have been hired by Amoco to solve the mystery.