I have had the POD HD 500 for about1year and half. Until 1 month ago, when ever I press firmly on tuner for a couple sec till it turns on, once I let my foot go,  it turns off repeately. So I have been tuning with my foot down the whole time but sometimes will just go off again. How can I fix it so that when I press it down, I can release it once it turns on like it originally did?