I have a 2008 Nissan Sentra. Here in the last few months, I have been experiencing a loss of RPM while driving on the highway. My car will drive fine for 3-40 minutes. After that, whenever I accelerate, my RMP will drop to 3.5 - 4 K. After 45 minutes, the RPM will drop to 2.3 - 2.5. As the RPM's start decreasing, you can her a high frequency whine coming from the car like there is some kind of inhibitor. At that point, if I stop the car, a foul odor will emanate from the vents. After the car sits for 30 minutes or so, it will be fine again. I took the car to the mechanic 3 times. The first time the replaced a wheel bearing. The second time they replaced the catalytic converter. The third time they replaced the emissions sensor that is in the catalytic converter. I took the car to a shop and it did not code. I am at a loss and so is my mechanic. Please inform. Thanks!