Poor chassis earth from engine to body and possibly to the battery could be the problem. Remove all negative 'black' earth wires from the battery and clean them up, not just on the battery side but on the body end. There should also be a link to the engine block, I seem to remember its a heavy wire (Or braid strap) bolted under a bolt on the engine to gearbox flange. Top tip.... Crank the engine with the headlamps on, if the lights stay on and bright its a poor engine earth. If the lamps dim to a glimmer, suspect the battery!
Good Luck Steve.
Steve Ball, November 2006