The ESP Guitar Company

I bought an ISP G String a few weeks ago, great pedal, really. Yesterday when we played with the band I used it for the first time with my modded JMP 2203 and at first it sounded very good, it did the job perfectly, no noise or feedback at all. But after 30 minutes or so it started to sound really muddy and when I disconnected the G String the sound was all good again, except the noise and feedback of course. 2 days ago when I played at home (Fame tube combo) I had the same problem but I never took the pedal away, I just stopped playing. Is it the battery that was bad or can it be something else? It sounded like the tubes were done or that I´ve played on max volume for 5 hours, very muddy and unclear sound... Gonna try the pedal tonight at home with a new battery and hopefully this won´t come back.