We have a fairly expensive ($800) KitchenAid dishwasher that's about 2
years old.

First of all, the manual says you don't even need to rinse dishes.
Just scrape off the big stuff and put them in. But the dishes come
out crusted with food particles.

So even if we rinse EVERYTHING first, it still happens. The side
rails of the sliding shelves are caked with crud. I used a scrub
brush to clean them off, ran a cycle of vinegar, and even used that
Dishwasher Magic.

Within a week, they are caked with crud again, and the dishes come out
filthy. We have to wash half of our dishes after running them through
the dishwasher!!!

Does anyone know what's causing this and what can be done about it?