The location of the SP should be one location higher than the RAM you used in your program.

The other possible problem is the register bank selector setting.
if your function changes this value, you have to restore it. Normally this was done by the compiler if you are using C, but you have to do it yourself if using assembler.

Another possible problem is using of registers such as DPH,&DPL which used in your function by not being initialized.

I dont know much about the assember you use,
but check if you changes the value of "EQU" or "SET", (or something smiliar) before different fucntions.


:-)
Lawrence