Cadillac Repair: 2000 escalade, loose connection, brakes

Question
Hi,
My friend has a 2000 escalade. It quits for no reason. Sometimes it will be ok. Last time she drove it she was on interstate @ 70mph. It died no steering,no brakes. Dealer replaced the key. No good. Dealer ran diags.. nothing showed. She is fearful of driving it.
Any help in this would be appreciated.
I suspect a loose connection, but know not where to start.

Thanks

Rob

Answer
IS or HAS the CHECK ENGINE light ever illuminated while driving the vehicle and or has any other lights illuminated or any messages from the message display center ?.

What work or diagnostics of ANYKIND WHAT-SO-EVER have been performed on the vehicle trying to fix this problem ?.