Hi Youssi,

If you haven't initialized the stack, then it should be at 07H by default. My first thought is that there may be some stack problem or overflow.

1) You may try by placing MOV SP,#07H in the beginning of your program (i.e re-initialize the stack, whether it is switch RESET or jump to 0 type RESET).
2) The problem may also be due to some software flags, which you had set prior to jump to 0, and when your software jumps to 0, they are not in the state as they should have been after a switch/power on reset.

I had a similar problem once, and it was due to stack.

Good Luck

IJAZ