While driving on expressway at high speed (and once on side street at 30mph) my car suddenly feels as the parking brake came on and speed drops down, not wanting to accelerate, RPM's go way up, around 4500, MPG drops to almost nothing. Also the car really does not want reverse, again like the parking brake is on. I can pull over, rest the car for 20-30 min. and then it seems OK. The first time this happened was about a month ago; did it twice in about one hour. Took it to Cadillac dealer and they could not get it to repeat the problem. There were no dash lights, no check engine problems. I had OnStar run diagnostics, and they found nothing. Everything was fine until this past weekend when we had the same issue. Again, car is at Cadillac dealer, and after 2 full days they can't find any problems.