I have 2 issues with my G-Dec Jr: The amp auto mutes randomly. It stays on, but mutes the output. Sometimes it comes back, sometimes I have to reboot the amp. Any suggestions to troubleshoot this? There is a low level, low frequency buzzibg sound that is always present when the amp is on. Does not matter what volume level or input is used. It is present even when nothing is plugged in and all volumes are at zero. Any ideas why this could be happening?