Audio Systems: Car Audio Problems, watt amp, rca cable

Question
I bought a car stereo for my car about a week ago from a flea market. I purchased a used cd player, and speaker set at 800 watts and a sub for the trunk, I also got a 800 watt amp. A couple days after they installed it i began having audio problems; They installed new speakers and they cut on and off while driving. Sound doesn't totally stop it will switch from the rear to the front and even side to side in the rear. I took it back to the installers and they didn't know what was wrong with it. I then took it to the experts, they suggested i check the wiring, i had that done but it still malfunctioned, I brought it back and they told me it was the radio, but it turn out the new one i purchased was not the problem. I then took it back to the installers and they checked and said it was the RCA cable, but he also suggested I get a four way amp for what I have no idea but he told me that might not solve the problem. I took it back to the other company and they checked the whole system out again and said that it should be fixed. But after starting back to drive I was furious to know that it was not. I have spent too much money on this and desperately need your advise. It only does this when I am driving, breaking, or making a turn. Can you help me?  

Answer
it is something in the wires there is nothing else that can do someting that crazy

or it is posible that the speaker are touching ground and makes the amplifier turn  of that channel and only the rear play or sometimes it happend the other way

look at the speaker connection ,see if the use connectors or they just put the wires there and check if the speaker metal is touching the metal of the car

good luck