Why does my dishwasher only drain intermittently?

My dishwasher intermittently drains during its cycles , if I empty the water from the dishwasher by hand and then continue with the cycle it finishes the programme. It can then run further cycles for a couple of days fine, but then once again will not drain. The pump has been tested with a multi meter and is reading fine. There are no blockages in pipe work and the filters etc are clean all of this has been checked. This has been ongoing problem for nearly 3 weeks .