Originally Posted by vr1000
Last year, I bought my first road bike in 20 years. A Giant OCR3. I have about 500 miles on it and its ghost shifting. That tells me its time for a tune up. What I want to know is it worth it to buy a 9 speed Ultegra rear derailleur and bolt it on. Will it improve the drivetrain or should I just go with Sora until I get my next bike?
I've been cycling for less than a year, but one of the things I'm glad I did early on is learn to tune my bike. I started riding with a 15 year old 'hybrid' Giant that I've had since before I could drive. I rode it out of tune, and learned to tune it myself, with the knowledge that if I broke anything...well, I wanted to get a new bike anyway! I did screw some things up but managed to fix them, and rode that bike for about 400 miles until I knew exactly what I hated about it. Then I got a used Giant OCR2. There are some things I don't like about it, and in a couple of years I'll probably get a better bike.
As for your problem, I'd be surprised if you've actually worn anything out in that many miles, so it's probably just breaking in a bit. You didn't mention whether you've damaged it (wreck it, get the cage caught in the spokes, etc), so I'll assume the RD's OK. You also didn't mention what kind of shape you've kept the bike in - if not the best, give it a good cleaning and make sure the chain doesn't have any frozen links.
If it's a new bike and still in good shape, it just needs tuning! If it's autoshifting, this probably means there's some friction on the cable at some point where you don't want friction. Worst case, this would involve replacing a cable, but might be solved with some lube. If it's simply skipping gears when you do actually shift, you can probably fix it with the barrel adjusters.
Again, I can't recommend highly enough the value of learning to diagnose your own mechanical problems, for a couple of reasons: 1) it'll be pretty expensive going to the LBS every time you need a little tuning, and 2) things may happen when there's not a LBS around. And in any event, you definitely can't go buying a new RD every 500 miles!
Good luck. As mentioned, if you have more details as to whether the ghost shifting is front/rear, up/down, with/without shifting, and with/without chain slip, that would help. Oh, and check out Sheldon's site as always, here's his article on derailer adjustment: http://www.sheldonbrown.com/derailer-adjustment.html