Need helps from experts!

Recently, I designed a 8052 system with DS89C420 CPU on SINGLE LAYER PCB (easy to make by my own), and it was properly operated as expected. But during testing with simulated noise (RFI, sparks manually generated by magnetic coil near by the board), the CPU run away.
Since my design does include internal(DS89C420) & external(DS1238) watchdog, although the CPU will back to normal operation after few ms, but this is out of my tolerances.
My PCB design does include decoupling capacitors (0.1uF) on each IC, unused area filled with ground, crystal near to OSC pin and etc.
But with AT89S8252 in the same board, the chances of CPU run away reduced.
I suspected that the DS89C420 very subjective to noise due to its processing speed, Is it true?
Could anyone advices me on how to solve this problem? I need DS89C420 to be my CPU due to its processing speed.