DELAYED INTENSIFICATION II** (SER ONLY): Beginning on day 29, patients (except patients with CNS3 disease) receive 8 daily fractions of cranial radiotherapy. All patients then receive vincristine IV, dexamethasone PO or IV, doxorubicin hydrochloride IV, study drug IV, cyclophosphamide IV, cytarabine IV or SC, thioguanine PO, and methotrexate IT as in delayed intensification I. Patients who were initially diagnosed with CNS3 disease receive cranial radiotherapy on days 1-5 and 8-12. Patients then proceed to maintenance therapy.