Sometimes if the RCA cables aren't strain relieved properly when they are mounted, the weight of the cables plugged into the RCA jacks on the amp can cause the solder joints on the amp to fatigue. This is a big problem on cheap amps where the RCA jacks aren't well attached to the amp case so the case will take the load.. Hitting the top of the amp and having it work properly is a sign that something is wrong with your amp, usually bad solder joints inside. You've heard that mounting an amp to a sub box is a bad idea, right? This is the same thing that happens to amps on sub boxes, the vibration and component weight inside starts turning solder joints to powder..