 DELAYED INTENSIFICATION PART 1: Patients receive imatinib mesylate PO QD on days 1-28, methotrexate IT on day 1, dexamethasone PO BID or IV on days 1-7 and 15-21, vincristine sulfate IV over 1 minute, dexrazoxane hydrochloride IV over 5-15 minutes, and doxorubicin IV over 1-15 minutes on days 1, 8, and 15, and pegaspargase IV over 1-2 hours on day 4 in the absence of disease progression or unexpected toxicity. 