I'm experiencing a weird issue on the green channel that's very sporadic. The amp will sound great for a while, both the green and red channels. Then, suddenly, when switching from red to green, the volume on the green channel drops significantly. But sometimes, it will also increase a lot and be distorted. Meanwhile, the red channel still works great. Sometimes, turning the amp off and back on fixes the green channel for a little while, but it always seems to come back again. This happens when using the Marshall footswitch to change channels, or using the button on the amp. Where should I start to look? Tubes, circuitry, somewhere else?