Hi Caerus. As you don't mention the technical specs of your bike, I'll just address the basics to check for. 1. Chain being too long will only matter when the smallest chain ring is in use. 2. Older dual pivot Shimano rear derailleurs has more free movement than the single pivot versions of Sram or the newer Shimano range. If you have the older versions, which I doubt, then an upgrade could fix your problem. 3. Rear hub free wheel(body), this could be worn causing extra slack on your chain, or if this was serviced recently, it may be a bit sticky and not as "free" as it should be. Easy test, lift rear wheel off ground, pedal in normal direction with one hand, then while wheel is still in motion, pedal backwards while observing chain line slack on chain stay. Hope that helps, if it doesn't, find a new LBS to help you. Regards
