I have a Randall G3 75w G3 combo amp. I have a problem with the volume. When I play the volume will suddenly get very loud then a bit later go back to normal volume. I have tried different guitar cords. I was thinking maybe it was related to where I was plugging in being on a power strip with my pedals. I ran it stand alone being the only device plugged in and still have the issue. I have noticed I can cause the change by putting some weight on the top of the cab. I suppose I should just open up the cab and take a look. What do you guys think it could be?