This sounds like possible trouble in your amplifier and/or wiring.

First diagnostic step is to hook up a known-good portable CD or tape player to the amplifier inputs, completely bypassing the head unit and all cabling leading to the amp.

If that's OK, then start tracing backwards from there to locate the fault.

Note: Do you own a Mk1 or a Mk2? My Mk1 player's docking connector needed to be cleaned regularly in order to be in tip-top shape for all connections.