audio volume pls

i have fitted in a pioneer sterio into my car which works fine, until i go over a bump, then the volume cuts out, although the cd is still playing. i then put my cd in to see if the volume works again and still the cd plays but no volume. then if i turn my engine off and on again the volume comes back on but only sometimes this does not always work. Has anyone got any ideas on what the hell is going on is my cd player faulty or is it a cable. if a cable has come loose i cant understand ehy it plays when i turn the engine on again sometimes????"mmmwoah

check all the connections for your  speakers, it sounds like a connection is loose and shorting out somewhere.  check the connections behind the radio and at each speaker.  did you replace the speakers as well?  if you did install speakers, make sure that the terminals arent hitting any metal behind the speaker.