if all of what you said is true, then return it i guess..... don't know what else to tell ya... Maybe your HU is going out....? If your confident, it's not your HU, then i guess it's the amp....same hu
one last thing to do, is to see what voltage your getting at the amp from your battery upfront. Do this by placing the Leads on the Battery & ground terminals of the amp, set the DMM to DC voltage, and see what you see..... should read over 13v with the car on