2006 Cadillac DTS cruise control problems

I turned right out of my place of employment. I was less than a half block from the light. I pulled into the center lane intending to turn left at the light. Suddenly my car accelerated rapidly. I slammed on the brake so hard my leg hurt for days. The car accelerated more and the brake did not stop it completely. I rear ended a large pickup. I went back to my office and contacted cadillac customer service immediately. I requested they check the "black box" computer module.

( just bought this used car , first time driving it home from dealer) speed control was set at 70 MPH on I-88 started down a steep hill, speed keep going up . I keep my foot on he brake as the speed was up to 85 MPH . The car never slowed down. I have had cadillacs beginning in 1991, never had a car do this before, if you are on a flat road doing 50 MPH, take your foot of the gas the car will start slowing down after you drive 3/10 of a mile and than slow very slowly.