Try Acid's solution...

I think it will most lickely be the connection of the cartridge/ Shell to the arm... sometimes the pins get really oxidized... also maybe of of 'em is stick hence the on/off contact.... clean it out (olso the pins in the Headshell/ Cartridge)... make sure the head is crewed on tighly (CAREFULL do not overdo it)...

let us know,

Z