Whether it is because power glitch at vcc causes some malfunctioning. Or it is because power glitch causes reset pin to get activated and then not recover properly? Or it is EMI / RFI causing the havoc by modigying registers or SFRs just internally

any or all three above.

1) use a good power supply
2) put the thing in a steel box (make sure you use one big enough to tahe care of the heat.
3) put ferrite beads on all wires at the place where they enter the box
4) put 100nf capacitors across all chips.
5) put a 4.7 or 10 uF tantalum across the '51
6) use a 4 layer board with ground and Vcc planes
7) opto-isolate all signals
8) use differential serial communications (422/485)
9) get rid of each and every ground loop.

Erik

Erik