Remove both pickups and blow it out by firing air in through one of the pickup windows. If you haven't got an airline you can try blowing it out with a rubber tube or pushing a tube in and sucking but make sure you use a tube which is smaller than the brush or you could choke. Check your slip ring for carbon traces and clean it with some fine emery on the end of a rubber tube pushed in one of the pickup windows. Make sure you are not using poor brushes - rub them on some white paper to see if they make lines like a pencil; if they do, bin 'em.