Chasing an alignment problem only on 6 and 7 cog
8 speed cassette. I've adjusted the RD (Shimano Tiagara). Everything is right except the 6 and 7 cogs. As I turn the pedals at "medium" effort, all seems well. If I turn the pedals fast and shift either from bigger cogs or smaller cogs, I get definite noise when shifting into these two gears. If I keep going, it'll get quiet again going to 5 or 8 and then all the way back to the #1 largest cog. If I am sitting on 6 or 7 and turn the pedals slow, every once in a while the RD will twitch.
I've tried barrel adjustments and can't seem to get rid of it. Any advice?