Could be. But I've developed a 2 channel TRIAC based phase voltage regulator for inductive loads based on AVR 90S1200 with 433MHz remote control option. Can say, works perfectly OK, but I also had some problems like you mentioned at the beginning.

I think that the problem is in the system design, not in the controller. Take care of proper and decoupled VCC, short signal paths that are connected directly to the controller, reset line is very sensitive to EM fields. VCC decoupling capacitor 100nF should be located as close to uC as possible. Take care to the GND line. Noise intensive loads (like TRIACS) should be connected to one point with separate signal grounds (there should be no signal GND wrapping over the PCB). Make your PCB as small as possible etc.

One can't believe, what careful system and PCB design means to the system stability and reliability.