I had the same problem with one of my project.
Peter is quite right, I replaced my 89C2051, which has 128bytes of RAM with 89C52, has 256bytes, and initialized Stack pointer to 7Fh. so My stack will start at 80h, which is the next location after 7Fh as 8052 stack inc by one before storing any data to it.

My problem was solved, I suggest you to give it a try.

Rauf