Transmission Gear Slipping problems of the 2009 GMC Yukon

2009 Yukon xl has died 3 times while driving. Once was in reverse. There is no warning. I've taken it to the dealership, they couldn't "reproduce" the problem so nothing wrong was determined. The only thing that quits is the engine (which takes out the power steering so I cannot control the truck for long and not at all in a turn); the air and radio/dvd player stay on, the lights (in - on dashboard and outside) stay on and no check engine light or any light that shows a problem comes on. The first occurence was in August of 2010, I was in mid turn, with my 4 children in the truck when I suddenly couldn't complete the turn. It died in the middle of the street. I couldn't turn the wheel or apply pressure to the brake. I put it in park, turned the key to off, removed the key, replaced it and restarted the engine (which started immediately). Onstar couldn't detect anything and the dealership couldn't get it to "reproduce" so nothing was fixed. In the past several weeks (July/August 2011) it has happened 2 more times, once while reversing (straight) and the other while again turning into a parking lot - this almost resulted in an accident. Both turns were right turns. The vehicle is being inspected, again, by the dealership with the same outlook; if it cannot be reproduced, there is nothing they can do. The transmission slips also but the dealership says they're not related. There are several incidents similiar to this online, as far as I can tell, none have been resolved. This is a very dangerous situation that will surely end up in an accident eventually. The first incident was at (about) 14,500 miles and last 2 were between 35,000 and 38,000 miles.