Don't know if this has been posted around here already (my apology if it was), but i just fixed mine by following below steps that i found on the intraweb and thought i'd share:

(when the chain is not properly tensioned - shifting quality drops, chain slaps the chain stay and drops off rings etc)

1-remove back wheel

2-remove both pulley and inner cage

3-now you need to remove the outer cage. First, take off the small allen bolt that stops the cage from going too far with the spring tension

4-once you removed the bolt, you can rotate the remaining outer cage, rorate it so the spring tension is at its weakest spot

5-Now you need to extend the derailleur by hand, you will see a tiny star head screw where the spring is hidden.

6- untighten the screw, it wont come out completely.

7- move the outer cage in back and foward, you should see the tiny screw come out while doing this.

8- careful, at this point the cage could come out and the spring as well. The outer cage needs to be in a precise position to be removed though. Inside you will find the spring.

9- remove spring, clean spring and clean the shaft on on outer cage. Regrease both.

10- look inside the place where the spring was installed, you might need a flashlight. You will see 3 holes where you can fit the spring, choose the hole thats gonna give as much tension to you cage as possible.

11- insert spring in choosen hole, put back cage, rotate to original palce then put back the allen bolt that will stop the cage from going too far.

12- tighten the star head screw

13-put back pulleys and cage, you're good to go.

When you choose your hole and replace the cage in its original position, you should feel a very big tension in the cage, some kind of tension that could snap your finger if you let the cage go. It took me quite a while to figure out how to do it, nothing tells you how to do that on the net.

It gave back some nice tension in my derailleur, maybe not as if it was new but much better then 2 days ago when the chain would go floppy.

let me know if you have any other question. Its a pretty simple operation actually, you should'nt have any problems, just dont lose any parts.