Please Help
 

I just bought a 2008 Cadillac Escalade six days ago. I drove it home from the dealer and had no problems for the first two days. Day three I get in my car to start it and it is completely dead. (it has a brand new battery). I opened the hood and wiggled the battery cables to make sure they were tight. I closed the hood and it started right up. Two days later I was driving on the expressway and when I pressed down on the gas to pass another car all of the electrical power in my vehicle was lost for a split second. It continued to do this all the way home as I pressed the gas with a little force. I wake up today and take it for a spin trying to recreate the problem and it was running find. Does anyone know what this may be? A bad ground?

Thanks in advance

Cont...
 

I also had my mechanic take a look and he said all the battery cables were tight and everything was clean.