Jon,
Try, in a few places, to make the program endlessly output the troubled location to P1 and use the scope. With a problem like this I often make the loops loop till say p3.4 is pulled low, that way you can try several places with one upload. With the info you have given, I doubt that an ICE with another brand of chip will show it. If, indeed it shows taht the mov ...,# does not work you may, as in a previous Atmel case, have to wait for revision 's'. How about trying with a Philips P89C51RD2 (no EEPROM, different IAP but otherwise the same).

Erik

Erik