Do you have an interrupts' vector table in your program? You put your code into interrupt table area, I think. You should to move your code to 30H or higher address:

ORG 0H
JMP loop

ORG 30H
... your code here ....

I remember also problem like yours, when I have activated watchdog. With simulator my program has been working fine but it didn't work with real hardware. Maybe you've made mistake like me (but if you are using pure 8051 - it hasn't Watchdog).