Could be a cheap amp, it could also be the wiring. If you're able, replace the wiring for the amp, including the power lines. Make sure that all connections are solid and secure. Ensure that the cables are good quality too.
It might be a dodgy connection in the signal or the power. If the problem persists, even with another audio source such as a portable CD player or radio, It's the amp itself or the speakers. If not, it's higher up the chain.
curiouser and curiouser
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--
Murray
I What part of 'no' don't you understand?
Is it the 'N', or the 'Zero'?