What i meant by reset circuit is a didicated ic something like MAX690 ( reset is active low need to invert it) I dont know about the 555 timer reset, is it good or bad.
How are you driving the AC motor, Mosfets, or IGBTs bridge , is it low or high power motor?
Search the forum, a guy posted a circuit diagram to make the 8051 work in noisy environment by adding a diode and 1000uf capacitor to the vcc pin(s) of the 51, he said it works.
motors are usually extremely noisy and you need line filtering at the input of the motors, see RS or farnell catalogues for line filters.

Mahmood