Can the Tensioner Spring go Bad on a Rear Derailleur? My bike chain has been "skipping", so I've gone through the obvious:


I replaced the chain
I replaced the smallest cassette cog, which was showing significant wear


After my replacements I noticed what the real problem was. Every once in a while, rather than the chain being pulled into the valeys of the rear cog, a chain link land on the tooth, and will not roll into the cog's valley and engage. Each subsequent link of the chain will land on a tooth. When the last engaged link begins to leave the valley, all the disengaged links will be snapped back into engagement by the rear derailleur.

What's going on here? Is my rear derailleur's tensioner spring just too old?
The tensioner spring is designed to keep the entire chain under tension, even after it passes the crank.  So, the symptom of a too-weak tensioner is slack in the chain between the crank and tensioner.  

It sounds like the distance between chain links is not quite the same as the distance between valleys in your sprockets.  This is not unusual if you replaced a very worn chain (because the sprockets would have worn to match the old chain).  It's also not impossible that the sprockets are worn not quite enough to match the old, worn out chain, yet still worn too much to match the brand new chain.  You may need to replace the rest of the sprockets in the cassette.