I got a Downtube FS 2007 on eBay at a very decent price. I just got around to trying it out today. The chain skips often in 4th gear. I realize Downtube suggests a look by a bike mechanic. Before I spend the $$$ on an inspection can someone tell me what might cause this and how I can make adjustments? I'd rather put more than a couple miles on the bike before going to a mechanic.