Dishwasher causes terrible smell in disposal – why

Our 15-year-old Asko dishwasher has its drain connected to the garbage disposal on our sink, and it in turns drains through a P-trap into a sewer pipe that has a "sure vent" (not vented to the roof or outside). When the dishwasher runs, a horrible smell erupts from the disposal. The inside of the dishwasher does not smell bad.

The plumber says everything looks plumbed correctly, and as far as he can tell the sure-vent is still fine.

We clean the inside of the dishwasher all the time, cleaning all the filters we have access to, and cleaning out the gunk that accumulates around the inside edges of things, it doesn't seem to help.

We've replaced the drain hose and verified it has the proper "high loop".

The most recent plumber declared it was the fault of our disposal and replaced it, but that didn't help.

I'm trying to solve this by process of elimination:

• If the sure-vent was stuck open, we would have sewer smell in the cabinet all the time. We don't.
• If the sure-vent was stuck closed, we would have other drain issues. We don't.
• If it was the filters and stuff inside the dishwasher getting clogged with food particles, the inside of the dishwasher would smell bad. It doesn't.
• If it was plumbed incorrectly, one of the several plumbers we've had check it would have seen it. All of them have said it's plumbed just fine.

Sigh.

The only clue I have, is that the dishwasher is having some mental issues. When we run it on any wash cycle, it counts down to 1 minute and then holds there forever. We've just learned that we need to keep an eye on it, and when it hits 1 minute we tell it to go to a rinse cycle (which takes 3 minutes) and that finishes successfully. I suspect (though cannot prove) that the heating element is not quite as powerful as it used to be, and perhaps the dishwasher is waiting to reach a certain internal temperature before calling the wash cycle "complete". And unfortunately, the dishwasher is old enough that a replacement heating element is not available.

Is there a significant amount of plumbing between the interior of the dishwasher and the drain, places where rotten food could sit and rot and wait for the next drain cycle, but where their smell could not reach the interior?