Exhaust smell entering cabin at high RPM/load on engine

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I have only had this ML55 for about 6 months. I bought it with 37K miles onit. It is a 2000. From early on, there has been an exhaust smell entering the cabin when the engine is revved up and their is a load on the engine (like accelerating up-hill from 50 to 80 MPH+). The dealer reviewd the issue 3 times and found nothing wrong, stating that thye couldn't reproduce the issue (even though it happens everytime you accelerate to highway speeds up an incline ). Finally the dealer's service foreman agreed to take a test drive with me. I drove with him in the vehicle for about 10 minutes, and sure enough, he smelled it alright - and had to roll down all the windows. He then took the vehicle in for service and replaced the CATs, O2 sensors, and some of the exhaust as well. This vehicle is a starmarck vehicle with a 3 year extended warranty for a total of 4 more years of warranty or 100K miles. They returned the vehicle to me and I test drove it again. The issue is still present. I just drove it for about a 1.5 hour ride on LA freeways, and ended up getting nauseaus and dizzy. They then took the vehicle back in and stated that they can't reproduce the issue. So I am on my way back to give them another taste of this. Does anyone know what it may be? What are my options in this case? This is an unsafe vehicle and based on the foremans last reaction, there is no way he would put his children in that vehicle.

I actually had one of the service advisors tell me not to drive it hard and roll down the windows. WHAT THE HELL IS THAT!!!

What concerns me is that the smell only comes through when there is a high load on the engine. The smell is exactly the same as when you get unburned fuel in your exhaust, which usually occurs when driving hard up-hill. However, I can close all of my windows and the smell pours in from the drivers side floorboard area. Therefore, it cannot be coming from the rear of the vehicle.

Any ideas?