possibly a bad earth connection. a set of clocks as Mitch says is an easy check but could set you back around £100, obviously depending on mileage they show. would only show if clocks are at fault...
you are looking at a 15 year old bike so could be down to corroded connections or terminal connectors breaking up,
this is why bike electric repairs can cost, breakages in looms, dirty connections, aged, heat damaged, vibrations are all problems on older bikes.
I could tell tales of cars and bikes where simple faults can cause headaches and tears.
