I have been playing a MM Strignray 5 for a while but I have a problem. In a rehearsal or on stage, my guitar volume seems to get lower after playing for about an hour. Or it slowly gets lower in time but it will be distinctive level after an hour, I am not sure. And the tone seems to get a little muddy at that time. Especially my G and D string volumes becomes noticeably low. After an hour or so I am struggling to hear my instrument while other instrument volume levels stays the same. I have tested it with fresh batteries but get the same result. I could not find the source of the problem. Anybody can help me? Thanks.