Washing Machine Agitator slipping?

My Simpson Aquarius 706 Washing Machine seems to Have agitator which is slipping. It moves much slower while agitating action is on although it moves in both directions. Because of this, the total move is also only about half a turn or less in each direction and the clothes do not get agitated enough to clean properly. If I put very light load then it seems ok. All other actions such as drain and spin look fine. Is there some kind of Clutch Or Belt which can be replaced to fix this and is it easy enough to fix. Thanks.