Cruise Control problems of the 2008 Infiniti EX35

I have an ex 35 infinity that has Smart cruise control. I had been using the cruise control for the previous approx. 5 hrs while I was driving on the expressway coming home from FL at about 75mph. , the cruise control suddenly stopped working and started flashing with limited power going to the car. . This resulted in a sudden deceleration of the car and I almost was hit by the car behind me. This happened 2 more times-with me again starting the cruise control and the resulting deceleration. I did not use the cruise control for the rest of the trip. I had experienced the same problem a year ago. Both times I took the car back to the infinity dealer. The 1st time, I was told that the sensor was dirty and they cleaned it-but really didn't find anything. This time I was told that they did notice that there was a cruise control stoppage in the electrical data-but that again the problem was that the sensor was dirty. I had just had my car washed and found this explanation strange. This dirty sensor is under the front grill. It can not be cleaned by a car wash-nor is there any warning that the sensor is dirty and that the cruise is not working properly. This "intelligent" cruise control also slows you down suddenly at times when you are trying to pass another car due to the side sensors. I have been told by the infinity dealer that there is nothing that can be done. I have no idea if the cruise control, is now working property. I am very hesitant to try it again for fear of an accident. It is extremely scary to be going 75mph and suddenly your speed drops unexpectedly. Please try to help me.