Arduino and MPU-6050

There is some problem with this sensor reading...

I programmed that LEDs should strobe if g-force > 1. After arduino starts sensor is too sensitive, it's starting strobe if i turn it ~20 degrees. But when i start Serial Monitor it works normal and starts strobe when sensor turned in 90 degree(~1G).

Can anyone explain it?

  accelgyro.getMotion6(&ax, &ay, &az, &gx, &gy, &gz);

  float gForceY = float(ay) / 4096; //Up to 8G

  if (gForceY > 1 && !isStrobeFast && !isStrobeSlow ) {
    isStrobeFast = true;
    strStart = millis();
  }


  if (isStrobeFast)
    isStrobeFast = StrobeMs(70, 8, strStart, isStrobeFast);

When i setup range up to 2G in MPU6050.cpp it works fine..
http://forum.arduino.cc/index.php?topic=339349.45