You must verify if the watchdog is enable and if so disable this function
If you can disable the watchdog, it is worthless. The 669 watchdog can, fortunately, not be disabled.
and save all interrupt in stack and restore it after the calls.Try to save the R1 registers and reinitialize all the register (r1,enboot) for each write.
Here we go again, the IAP routines are suppoosed to be followed by a reset!!. The designer of the ROM code is free to use any register SFR or whatever (s)he desires and what that is can change from iteration to iteration.

Erik