Hey, I've had an AD100VT for about 6 years now and have been having noise problems for about 2 of those years. Though the sound worked perfectly through headphones no matter how long played for. I had enough of the sound cutting out so I did what i could to look into it. inspected the solder joints, made sure any vents weren't blocked up, and then changed the valve. this seemed to have done the trick.
I sent a message before asking could it be valve causing the problem and the response was "No", which made me not even consider changing the valve for a long time. Looking at the valve, it didn't seem to have any defects or any difference at all on it.
Try chaning the valve.
I have now played my amp with the replaced valve for a total of about 14 hours, with one of the sessions about 6 hours without a glitch.
I hope anyone with the same sound problems tries replacing the valve and works for you.