There was a post similar to this in 2009 so I've copied most of my reply here:
The problem with your raspberries might be root rot. Its a Phytophthora fungus that lives in the soil especially where the soil is heavy and may get waterlogged in winter.
What seems to happen is that it lays dormant, then the raspberries start to grow nicely, then the temperature of the soil reaches a certain level and POW the fungus attacks and the plants start to shrivel and die. It's devastating.
If you have a look at the base on the canes you may see brown markings where the canes have started to rot off. If you dig them up, you may see that the roots are less well developed than you would expect and may see some signs of rotting.
Commercially they drench the soil with a fungicide to help prevent it, but I'm told once you've got it in the soil its hard to get rid of. So I dug up what was left of mine in that area, burned the canes and roots, and re-planted fresh canes in another more well-drained placed with lighter soil.
Hope this helps.