I am using an SM57 to record my guitar cabinet. At random times during a recording the volume level will drop signifigantly (like -10 dB) and randomly return to its normal level. It generally does not stay between the two extremes. Also the line can get a little noisy when this happens (not a lot like a loud hum, but noticable enough where it is annoying). I am not moving around or adjusting the cables, mics, or preamp levels when this is happening. I have tried to isolate the mic, cable, and preamp by trying different combinations, but I can never seem to duplicate the problem when I want it to happen. Sometimes I can play for an hour before it is a problem, and other times it will go back and forth more than twice in a minute. Every time I have tried to run tests using just the preamp, just the mic, or just the cable with other mics, cables, and preamps I can never duplicate the problem. Does anybody have any experience with a problem like this and what it could be? I hope it is the cable since it is cheaper to replace than the other pieces.