Seems like the problem was in old series:
In T89c51RD2 Page exist a link to a pdf document explaining how must work the reset circuit and says somethink like "don't use an rc or you can loose datas"

However in AT89c51RD2-ED2 says nothing about it.

I tested my system without any exeternal reset circuit about 50 on/off cicles and works fine but... 50 times in a 'clean' enviroment is not enought i guees!

I must test if i re-conect the external supervisor, the circuit can distinguish the source of reset.

I will become with news about my tests