I have same like this prob. in my 325i, it's RPMs around 500-600 only, and much emissions ( no smoke, just bad smill ) from the exhaust coming. No vibration in the engine every thing seems fine with hoses.
the car performance is seems fine, I'm not sure about it. but I feel there is somthing wrong with that low RPMs and that much exhaust emissions, which seems to be caused by misfire.
I'll replace the sparkplugs and see the result.
Tell us if you solve your prob.