Weird flicker

[The Master]

Hi. Ive been making a circuit and ive started getting trouble with lights flickering. The chip does use PWM but the flicker is something different. The chip will work fine then for no aparent reason all the outputs will turn on and off rapidly (about 10 times a second). I had this problem when i was testing but only if i started unplugging the mains supply slowly. Could this be a brownout? The chips stop responding to the serial input. Whats the most likely cause? Programming error? Dodgy circuit?

Edit: In my circuit i have 2 chips and they both did the same thing at the same time. Weird