The problem I'm having is that the derailleur is not pulling the chain down onto the smallest cogs. When I change the shifter up to 8, the cable is untensioned on the top tube but the chain is stuck on the 3rd smallest cog. Occasionally a bump in the road will knock it down another cog, but generally it's only going 1-5 (on the shifter numbers).
It appears that there is tension on the cable somewhere between the top of the seat stay and the derailleur itself. Where to I start to debug this? Does this sound like a cable guide problem, or do I need to reset the limit screws (which appeared to be correct)?
I've recently replaced my rear gear cable with a new one. I originally did not dial it in correctly, so while the H & L limits seemed correct, it didn't seem to be indexed correctly - chain jumping back and forward between middle cogwheels. A guy at the LBS took a look at it and, in front of me, reconfigured it from scratch. He had it shifting perfectly through all gears. I took it home, worked great on the ride home.
Next day, after about 35-40 miles I had some trouble with the cable. The plastic sheath had popped out of the guide on the back end of the top tube (4300 runs the cables across the length of the top tube and down the seat stay). I released tension on the cable (into lowest gear) and put the cable back into place.
Revelant bike spec: Trek 4300, STI integrated brake-shifters, Deore front mech (2-3000 miles) , Alivio rear derailleur (3-4000 miles), new Shimano 8spd chain, new rear Shimano 8spd HG cassette, original front gear cable (3-4000 miles), new back gear cable (~80 miles).