gearbox slips out of 5th when idling, vectra 1.8ls?

gearbox slips out of 5th when idling at 2thou,at about 50mph,i have been advised to top up the gearbox oil,which has now been done,slight rattle.when in neutral car ok as soon as i put the clutch peddle down slight rattle not to confident in driven it,please can you help as my wife needs it for work,cheers,Ant