Mountain Biking - Derailleur "skipping" WTF?

I had a small problem with my bike this week when my derailleur [SRAM x.7] cage ended up getting caught, in my rear sprocket, [chain broke :( etc etc] well, I replaced the chain and now the thing skips whenever I pedal hard, making it hazardous to do so. [when I pedal hard on a lower sprocket the chain pushes past the jockey wheel and speeds past the sprocket whereas it would normally just get passed on smoothly]

A brief look showed that the jockey pulley on the derailleur isn't as close as it should be to the sprockets on the wheel [shouldn't the jockey's teeth be around 6mm-1cm away from the sprocket teeth? it's about twice that.]

I adjusted the screw on the derailleur that determines the distance between the jockey pulley and the sprocket and that worked for about 10 miles. I bought a SRAM x.9 thinking the derailleur's knuckle-spring had been stretched, moved or something else and I'm having the same problem with the brand new one too.

The hanger for the derailleur isn't bent or streched at all, the frame is in perfect shape, I'm at my wit's-end trying to figure this out.

The shifting is smooth as butter and it's not skipping gears, it only does this in the bottom three sprockets when I'm pedaling hard.

Any ideas of what else this could be?

Check your chain for tight links. Happend to me.

How many cogs do you have on your casset (8/9)?

Any what size chain did you get (8/9)?

1. Stiff link in chain. This almost ALWAYS happens when you buy a new chain (well has for me). You will find the connecting link will be stiff, and you just need to 'ride it out'.

2. Cassette worn.

3. You may have the wrong size chain / no. of links. Take it to a bike shop and let them determine whats wrong with it I say.

If you replace a slightly worn/used chain with a brand new one, it will skip much more than the old chain. This is because the old chain stretches with use and the slightly longer links wear their own pattern into the cassette. Now, when you put on a brand new chain that is not stretched at all, it will skip over the worn cassette that was "molded" to fit the old chain.

You might have a few bent teeth in your cassette.

You need a new cassette.

I am insensed right now. I just found out what the problem is... the jackasses that installed my cassette installed the first two sprockets BACKWARDS! the teeth are ramping INTO the chain. I want to strangle them. >:[

Any ideas on whether this could cause any permanent damage to anything? Can I get them to pay for my new chain and derailleur?

Well if they installed it wrong then hell yes. Go back and DEMAND it to be fixed! :p