I use the 89S8252 EEPROM also in a lot of projects without problems.
I see a similiar problem on a forum
earlier so I tested permanent storing
until the 89S89252 stop it on to low power.
The only effect I found, was, the current loaction was erased (0xFF) but
not reprogramed.
And some times the adddress 0x0000
was also erased.
But I found never altered the code.
I used crc and fill the whole 8kB code
space.
So I think you have a single bad device
or your power was not good.
E.g. switching power off cause rapid
changing outside the operating level.
Please watch, that a transformer
can induct high voltage on switching
off and a regulator (7805) can let
pass it if fast and high.
A suppressor diode (ICTE5) can
avoid this.


Peter