I seem to be having a problem with my rear derailer. I have an 8 spead shimano rapid fire connected to a shamano deore derailer. When riding in any of the middle 4 cogs on the back the chain has a tendancy to jump either up or down a gear. Sometimes it reverts to its former gear, other times it stays in the new one. I looked at the Park Tool Co. site to research the problem and followed their advice on aligning the derailer. The problem is that when on the bike stand, there is no problem when I shift gears. It is not a worn chain or cog (I don't think) as it happens under all types of riding, uphill, flats, accelerating, etc. Can anyone offer any advice on what would be causing the derailer to shift on its own. Oh yeah, both parts were replaced by the lbs last week.