Shimano America XT 9 Speed Cassette

Honestly I would be very surprised if the rear cassette was the source of the issue. Not to personally attack you, but 99% of the people I encounter in our shop that have this issue ride while cross chained, mash down on the pedals instead of spin, and don't keep their chain properly lubricated.A few suggestions:1) If you're cross-chaining, don't. When you're in your low chainring up front, keep your rear cassette in a low gear. When you're in your big ring, keep it in a high gear in rear. This keeps the chainline straight, and it's the biggest reason people suffer from 'ghost shifting' and other plagues.2) If you're not riding with clipless pedals, seriously consider them. They encourage much better riding techniques that demand less energy of you, the rider, and also helps keep the drivetrain in much better shape. I personally prefer Crank Brothers products, they're lighter and easier to use than Shimano's SPD products. Check these pedals out, cleats are included with the pedal:http://www.backcountry.com/store/CRN0009/Crank-Brothers-Candy-Sl-Pedals.html3) Finally, lubrication is a total must with mountain biking (and even with road biking). I lube my chain before every ride, and use a proper de-greaser to clean my chain just about every other ride. I personally prefer Finish Line Dry lube, but all lubes are generally equal. DO NOT, under any circumstances use WD-40 as a lube. I don't know how available it is in Germany, but it is not a lubricant and never has been. It's a water displacer, and it ruins bike components.Hope you don't feel like I've railed on you, it's nothing personal. If you are for sure needing a new cassette (the teeth might be ground down from so many chains being mashed), I highly recommend SRAM products. They're generally lighter, cheaper, and better engineered than Shimano's products. SRAM's chains also rock, they allow much easier on-the-trail repairs with the PowerLink (and they're stronger, too).In regards to cassette sizing, it's a personal preference. An 11-32 cassette is standard on most mountain bikes, but an 11-34 gives you an easier gearing option if you struggle going up hills with your existing 32 tooth cassette.If you have any other questions, feel free to post them and I'll gladly throw in my two bits. You're always welcome to e-mail me at mattf [at] bikemonkeyranch.com as well.