So--my bike is finally assembled, the brakes are adjusted and working beautifully, all ready to go...almost. I went out yesterday for my first real ride and found that every five minutes or so, it would pop out of gear--ga-chunk--and leave me spinning my pedals until I switched gears and then it would go back to normal. I know this is probably a common issue of some kind with bikes this age, but I've never had it happen--I have bikes that pop out of third into second, etc. when I'm pedaling hard, but this just goes completely and when I'm just pedaling easily along. Can someone give me a clue before I trudge back to my LBS with more of my hard-earned?