12v LED Strip softly strobing when it should be off.

Thank you in advance for any help you can provide.

This project is simply fades in an LED light strip inside a display case as you walk up to it and fades it back off as you walk away. This worked for about a month, but now the light strip softly strobes or pulses when no input is present. The light continues to work normally when a person walks up to the case. An infrared motion detector with a built-in timer is being used to provide the input to the arduino board. The 12vDC source powers the Arduino UNO Rev3 and the LED light strip.

NPN MOSFET part number: STP55NF06
10K 1/8W pull down resistor on the input
330ohm 1/8W pull down resistor on the Gate

Code in use:

#define BUTTON 2
#define LED 9

int fadeAmount = 1;
int brightness = 0;
int delayTime = 10;
int buttonState = LOW;

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(LED, OUTPUT);
}

void loop() {

  buttonState = digitalRead(BUTTON);

  if ((buttonState == HIGH) && (brightness < 255)) {
    brightness += fadeAmount;

    } else if ((buttonState == LOW) && (brightness > 0)){
     brightness -= fadeAmount;
  }

  if(brightness > 255) {
      brightness = 255;
    } else if (brightness < 0) {
      brightness = 0;
  }
  
  analogWrite(LED, brightness);
  delay(delayTime);
}

. This worked for about a month, but now the light strip softly strobes or pulses when no input is present.

So the problem is likely to be a hardware one then.

Please supply a full schematic of your circuit.