Bosch Dishwasher Drain problem

Question

2 yr. old Bosch Dishwasher (SHE46) does not drain all the way about a third of the time. Sometimes does not drain at all. Most of the time it drains just fine. Hose is clear. nothing clogged around the impeller. Hose is connected to the garbage disposal & elevated 20'' above floor per directions.Any suggestions? Thanks.Mark