Read the huge 6sp thread.
From http://sheldonbrown.com/derailer-adjustment.html#chain
Skipping/Autoshifting
Do your pedals sometims jump forward when you pedal extra hard? This is a common complaint, especially when a rider stands up to pedal. Indeed, this dangerous condition is one very good reason for remaining in the saddle and spinning in your lower gears, rather than standing up and pumping in a higher gear.
Although jumping/skipping/autoshifting is often blamed on the derailer, it is only very rarely the result of a derailer malfunction.
This jumping may be one of two totally unrelated problems: skipping or autoshifting. The first step in troubleshooting this problem is to determine whether the problem is simple skipping or autoshifting.
* Skipping involves the chain jumping over the tops of the sprocket teeth under load. After the chain jumps, it remains on the same sprocket. This is usually caused by wear to the chain and/or the sprockets. This is most likely to happen on the smaller rear sprockets, especially if they are used in conjunction with the small chainwheel in front. This issue is addressed in considerable detail in my article on Chain Wear.
A form of skipping, not necessarily under load, sometimes also results from stiff links.
* Autoshifting feels just like skipping, except that after the jump you find that the rear derailer has shifted up to the next smaller sprocket. Autoshifting is commonly caused by a combination of frame flex and cable friction. The mechanism of this is explained in detail in a separate article on Autoshifting.
From http://sheldonbrown.com/derailer-adjustment.html#chain
Skipping/Autoshifting
Do your pedals sometims jump forward when you pedal extra hard? This is a common complaint, especially when a rider stands up to pedal. Indeed, this dangerous condition is one very good reason for remaining in the saddle and spinning in your lower gears, rather than standing up and pumping in a higher gear.
Although jumping/skipping/autoshifting is often blamed on the derailer, it is only very rarely the result of a derailer malfunction.
This jumping may be one of two totally unrelated problems: skipping or autoshifting. The first step in troubleshooting this problem is to determine whether the problem is simple skipping or autoshifting.
* Skipping involves the chain jumping over the tops of the sprocket teeth under load. After the chain jumps, it remains on the same sprocket. This is usually caused by wear to the chain and/or the sprockets. This is most likely to happen on the smaller rear sprockets, especially if they are used in conjunction with the small chainwheel in front. This issue is addressed in considerable detail in my article on Chain Wear.
A form of skipping, not necessarily under load, sometimes also results from stiff links.
* Autoshifting feels just like skipping, except that after the jump you find that the rear derailer has shifted up to the next smaller sprocket. Autoshifting is commonly caused by a combination of frame flex and cable friction. The mechanism of this is explained in detail in a separate article on Autoshifting.
