const int ledPin = 3;

unsigned long now = 0;
unsigned long timerA = 0;

void setup() {
  pinMode(ledPin, OUTPUT);
  now = millis();
  timerA = now + 128;
}

void loop() {
  // put your main code here, to run repeatedly:
  now = millis();
  if ( now > timerA ) {
    analogWrite(ledPin, rnd128() + 90);
    timerA = now + rnd128();
  }
}

uint8_t rnd128(void) {
  static uint8_t s = 0xaa, a = 0;
  s ^= s << 3;
  s ^= s >> 5;
  s ^= a++ >> 2;
  return s >> 1;
}