Hello Benny,
Yes, it has got to do with undervoltages, the capacitor+resistor+diode combination as shown in the data books of microcontroller manufacturers is usually not reliable, during brown outs that is a partial dip in the voltage at the downstream of the 7805, may not reset the micro properly and the micro will usually hang or worse it may corrupt the eeprom . I learnt this lesson way back in 1989 with a Z80 microprocessor.

In all my products even cheap products I have used the DS1232, this is a brown out protection device, more correctly called as the microprocessor supersvisor, with a watch dog timer, with this device you can also adjust the brown out trip point, but this device is showing its age, because the watch dog timer cannot be disabled, if your implementing the ISP in the 89C51RD2 then this device is not practical, I am now going to try the MCP810 which is another brown out device which is widely second sourced. Offcourse there is wide availability of such devices, google to get more information.

If your intention is to design reliable embedded product then you will have to spend time studying about brown out protection, reset duration etc.

Regards,
John.