I have my MP3 player hooked up to my cassette player using the cassette adaptor sold on Ebay and the like. Haven't had any problem until recently.

However, the sound to the drivers side speakers keeps cutting out, but does come back on for a second or two if I adjust the mp3 player jack in the mp3 player.

The speakers work fine when I have the radio on.

The problem is not the mp3 player or the cassette adaptor because I have tried a brand new mp3 player and a new cassette adaptor.

Any ideas?

Probably a kink/broken cable or a duff socket. Cheap cables often pack up just at the point of entry into the socket.....