...continuous reset

What does your Vcc look like on a scope?
>> 5 Volt DC ( Clean Vcc)

Vmax : 5.36 V Vavg : 5.123 V
Vmin : 4.88 V Vrms : 5.124 V
Vpp : 480 mV

Do you have 10-100nF directly at the chip?
>> Yes (directly across the chip) also bulk d-coupling on the board

"Old product" do they all do this?
>> No. only some of them (with P89C51RD2 on board)

PS: the same board with P89V51RD2 works fine and even the Atmel's AT89C51RD2 works fine.

is there a supervisor chip?
>> No, Only simple RC reset circuit is there. (3.3uF)

breadboard or PCB - how many layers?
>> It is a two layer PCB, and been in production from past 5 years.


Thanks,

Mihir