hi,
well, so it is time to ask you about the board.
Is that board you use, custom made? Has it reset via (R)C or is there a special reset chip installed?
I ask because there is another difference: reset pin of AT89S8252 has 50...300k pull-down resistance when one of AT89S52 has 10...30k only. So if you use just reset capacitor then increase its value at least at ten (x10) times. Hmm, I always preffer to use power SV instead this "magic" feature.

Regards,
Oleg