Could be the crankshaft position sensor. My 07 4.8 had similar issues. Then, finally, I walked out from work one day to go home and it would turn over but never start. Then, I tried again and I got a transmission failure message. So, I called a tow truck and when the truck got to my office, I tried starting it one more time and it started up without any errors. I drove it right up on the flatbed and turned it off. Dealer replaced the crankshaft position sensor under warranty and it's been good ever since.