A little more info is going to be needed. Does it run roughly in all gear combinations or are some silky smooth and others rough? Is the amount of "roughness" different if you up-shift to get to a certain cog then if you downshift to get into the same cog? That can tell you if your cable is too tight / loose and therefore under / over-shifting. You can try a little light finger pressure on the rear derailleur as well -- see if there's a direction you can push the derailleur and cause it to suddenly get "silky". Warning: don't push hard -- derailleurs bend easily.

If this is the first time the bike has been looked at it in eight years, you probably also want to make sure the pivot points are well lubed. You might just have a case of "derailleur stickies" which aren't allowing the derailleur to move as far as the cable wants it to.

One other thing to check: the pulleys should be in the same vertical plane as the cog the chain is currently in. Make sure that they're hanging straight. If not, you need to have it bent back before you have any hope of getting it to work smoothly.