Two weeks ago, our year-old Frigidaire dishwasher stopped draining; it left
a big pool of dirty water in the bottom after the full cycle and didn't get
the dishes clean. During the drain part of the cycle, it made a vibrating
noise, as if the exhaust pump were running but with no water in it, just
air.

I removed the water; the plastic filter which covers the well at the bottom
wasn't clogged. I removed the filter and there was nothing blocking the exit
port at the right side of the well. I pulled the machine out of the cabinet
and took the exhaust pump and lines apart. When apart but connected to
power, the impeller blades spun well. There was no blockage between the exit
port in the well and where the tube connects to the sink's waste line. I put
it back together and it worked perfectly. I figured that maybe it just
wanted some attention.

Two weeks later, the bottom is again filled with dirty water and the pump
makes an empty, vibrating sound during the exhaust part of the cycle. I'd
rather not take the darn thing apart every two weeks. Any ideas on what
could be causing this?

Paul