When in neutral the forward gear is still engaged, when the revs. are increased the speed of the prop shaft also increases. When the engine is on tick over if I put my foot on the prop shaft the prop shaft stops turning. When either the forward or reverse gear is engaged they work normally. Since gear box oil change the engine has run for about 400 / 500 hours. Please advise on how to solve this problem and if I do nothing will it get worse ?