new cassette

First thing I would do is remove and inspect the hub to make sure the cog itself isnt slipping. If you cansee evidence of a worn gear just replace. I went to 12 as my higher and didnt look back. I loved the gears and work great for me. Good luck

So how exactly does the gearing work?  The lower the number the harder it is to crank, but the faster you can go, and the higher the number, the easier it is for climbing hills?

The cogs on the cassette is slipping?! This can only happen if either the hub is shot (and that's slipping) or the individual cogs has broken off the carrier of the cassette body. Either way, you need to take that cassette off and look at it.

Benefits of gear ratios... a 12/23 cassette has the benefit of a fairly close ratio, which means that when you shift from one gear to the next, you have fairly small jumps, which means that you can find the optimal gearing for the terrain to maintain cadence. The drawback is that often a 23 largest cog is a bit too much for most of us... we need at least a 25 cog for most climbing.

For me, I've found that a 12/25 with a 50/36 front (compact in other words) gives the benefit of having an easy enough gear for pretty much all climbing (did Mt. Ventoux with this), yet a high enough gear that I race with it (I road race as well as duathlons).

Also, you need to find a new bike tech... if they can't figure this out, then it's a real problem.

Something is slipping.  In gears 7 and 8, whenever I am pedaling hard it is slipping.  Several techs have not seemed to get it to work.  I think maybe b/c it is only when I am pedaling hard, if your just going through the motions, no problem.

That doesn't sound like a problem with your cassette...you need to take it to a tech who is not so lazy and can pull the cassette for you and check for the problems in the wheels the other posters have referenced.

Good luck.

Mike

I rode with a guy last year start started to have this type of problem on our rides.  Turned out to by his hub and had to get a new one.  For the price he paid the shop to just replace his hub he could have found a decent hardly used OEM wheel on the classifieds. 

Gears, most front cranks are 39 tooth small ring and a 53 big ring.  The cassettes can vary , e.g. 11-28, 12-21, 13-25, etc.  With a 39 ring and at 23 rear cog you will turn the tire 1.695 times (=39/23) for each revolution of the crank, this end of cassette would be for climbing, etc.  With a 53 ring and 12 rear you will turn the tire 4.416 times, think downhill/tailwind speed.  The more you spread out the cassette gears(e.g. 12-27 vs 12-23) the more spacing you'll have between some of the gears on the last 3 or so cogs.  Visit http://www.sheldonbrown.com/, lots of great info there.