Sound to me like you have a slipping clutch. You clutch disk's tooth have been damaged. Its normal on the car assuming it is the stock one from 1998, 9 years is enough for the clutch disk. When you are moving the grip is fine since the speed of the dish is up and gripping slightly better. When stopped, slow engine speed makes the disk dis engage with the engine a little so your engine will shake a bit. I suggest if you don't want to change it now, change it soon, if not, when stopped, shift into neutral then you shouldn't have the problem.