I realize this is an old post but for others reading the post with the same issue, I have the same problem. The workaround is to either lower the volume on the guitar or use an effect pedal between the guitar and the amp. I'm no expert but from what I've read some guitar pickups are overloading the amp and it is invoking some protection circuitry. The effect pedal must protect the amp from an input overload. Interestingly my guitar that I have had since the mid 1980's does this behavior with the amp but it didn't used to. Like I said, I'm no expert but I hope this helps.
