Dishwasher drains sometimes, but not always.

I've noticed lately that my dishwasher will complete it's full cycle and the water will have drained the completely about 50% of the time, but the other 50% of the time there is about a 1/2 of water that wasn't drained. I simply run a rinse cycle and it drains fine. Sometimes I can go a week without having an issue. I've checked the line running from the dishwasher to the disposal and didn't identify any clogs. The hose from the dishwasher to the disposal also seems to have an adequate "high loop". I've also checked the filter and cleaned any gunk in the bottom of the dishwasher (very little). Any ideas on things I might be missing? This is a fairly new dishwasher (about two years old).