You are probably overloading the '51. read the datasheet for the specific chip you are using for Iol max. If you want to drive LEDs directly, you must resistor limit the current to a value below the Iol max. Another factor is the heat disapation at several pins running at Iol max, you must multiply the voltahe at the Iol you have with the Vol specified for the '51 at that current, add it to the "basic" power and make sure you stay below the (package dependent) maximum power.

Erik