[toggers]

My chain will hop occassionally no matter what rear cog it's on. It doesn't happen every revolution of the cranks, either. It will run smoothly for 2-3 revolutions, then rise up as if it wants to go to the next bigger cog, or as if it's not seating on the current cog. Also, the pulley arms will jump a little. I've fiddled with the screws on the derailler, and it runs from cog to cog alright when I switch gears. I've tightened and slackened the derailler cable; it doesn't seem to matter much. Could it be the chain or pulleys? I've adjusted deraillers on other bikes, but this one is giving me troubles (Trek 1000c).

[photosbymark]

There is a barrel adjuster that might be your issue. Not sure where it is on the trek, but many are right up near the handles on the shift cable. The screws on the derailleur are limit screws. One sets the limit for the big cog side and the other for the small cog side. If its not an adjustment the next thing to check would be chain wear and from there it would be internals in the rear cassette or freewheel, but I am going to guess your barrel adjustment will correct it. In the old days the gears would stop trying to shift when you stopped messing with the lever. You were the indexing. Now its more complex, and I guess its better. Yet at times I still miss the old ways.

[CTD50]

Sounds like a chain issue, not a shifter/derailleur one. My first guess is a tight link, second guess is a twisted chain.

Since you've messed with the limit screws, be sure they get back to where they need to be....

[Noel]

I'm going with the tight link or worn chain theory, chances are if you haven't recently taken off and reinstalled your chain then it is a worn chain and not a tight link. It could be, but probably isn't, worn pulleys. Never try to adjust the derailleur by turning the limit screws, you'll end up un-shipping your chain. Rear derailleurs have a barrel adjuster right where the cable is attached to the derailleur.

[Crinklecutter]

Bent derailer and/or worn chain and cassette? If the cassette is worn along with the front chain rings replacing just the chain probably won't help.

[RoadHollow]

Since you have posted in in section of the forum .And If the adjusting the cable hasnt worked ? I would suggest taking your bike to a trusted mechanic and then take a "class on basic bike mechanics" .Or if you have a close friend who wrenches . You can do a lot of costly damage by messing around . Trust me I had !!! The shifting system is a very basic and simple mechanical system that once demistied will save you a butt load of money and make trail tuning simple on the fly stuff .And then you can pass along your knowledge !

[qmsdc15]

You have a stiff link. Click here for the fix.