hi,
just put those discrepancies here and perhaps people say what is what.

BTW, have you the last AT89C51RD2 datasheet?
And of course read this errata about "undocumented features" of AT89C51RD2. There a lot of differences with original T89C51DR2. For example, next errata notes may cause incorrect work of AT89C51RD2 when it is used instead T89C51RD2 without correcting a software:
- Timer0/1 – Extra Interrupt;
- C51 Core – Bad Exit of Power-down in X2 Mode;
- PCA – Incorrect Behavior with CPU X2 Mode Bit of HSB.

These errors were not presented in T89C51RD2 so check your software again and again and then say big thanks to Atmel for such piece of errateous chip(s).

Regards,
Oleg