A lot of people post this question of sound just disappearing after e.g. 3 minutes of play, while lights are still on, the LCD panel is on, headphones still work. I've taken my amp to the repair shop (no official one in Slovenia) and they sad they corrected a couple of bad solderings. But that actually didn't solve it. They claimed they have played radio on it for 3 days without problem. Although I put in my guitar and the same thing happens again.
So then I swithched off the bluetooth connection with my iphone and voila, the amp sound no longer dies after 3 minutes. I am not sure where the problem is but I hope that you haven't yet thrown the amp in the garbage bin.