2005 CTS 2.8l: All six cylinders misfiring

I'm new here so I will try to be as detailed as possible.

1) One week ago the engine started to hesitate and briefly rumble when I accelerated. The CEL would flash briefly for 30 seconds and then nothing. Sometimes it wouldn't flash at all or hesitate and everything was fine.

2) The next day, I took it to an Advance Autoparts to have the codes checked and nothing showed up on their scanner

3) Today the CEL came on (no flashing) and stayed on and there is an intermittent rattle in the engine. Car starts fine with no hesitation but still hesitates on acceleration

4) Today I took it to Autozone and it was scanned again and it gave the following 7 codes

P0300- misfire detected random cylinders
P0301- misfire cylinder #1
P0302- misfire cylinder #2
P0303- misfire cylinder #3
P0304- misfire cylinder #4
P0305- misfire cylinder #5
P0306- misfire cylinder #6

5) Car is 2005 CTS 2.8L, 101,550 miles

I don't really know what to do next and the car's certified warranty recently expired and money is a problem. Thanks for your help.