[Digital_83]

Check the "air circulation" setting on the climate control panel; the far right button, between both temp dials [has letter A-M and an arrow sign].
Your car's HVAC system could be periodically capturing some outside air when set to "A" mode.

edit: I just re-read what you wrote OP - does this happen in normal traffic conditions ? I mean, getting to a choking point is rather alarming ... I can see that level of exhaust odor happening when you're in near dead traffic idling in a tunnel or behind of a diesel truck, but if it's just you and your car ..