Hi, Carlos
For hardware design, it is better to add pull-up resistor to reset pin. As you know, there is a internal pull-up resistor in our MCU, but it is only a weak pull-up. So it is recommend to add external one. If your software can run well most time, only will cause reset sometime by unstable reset signal. I think it might cause by non-external pull-up resistor.
Hope my reply can help you.
Best Regards
Paul