Chain slipping in cassette when under load, could a different type of cassette work better?
The chain will typically slip under the following conditions:
 1. the chain is worn out (this can be measured with a cheap tool,
    or by your LBS).
 2. the cassette is worn out 
 3. one or more of the chainrings (the cogs on the crankset) is worn out 
 4. the chain and/or cassette/cogs/pulley wheel are dirty and full of oil/road-grit/gunk.

These can occur simultaneously - a dirty chain causes rapid chain wear, a worn out chain causes the cassette to wear out.  If you have been riding with a slipping chain for long, you are very likely to have worn out both the chain and the cassette and will have to replace both.