I have a 2003 Chevy Mont Carlo SS 3

I have a 2003 Chevy Mont Carlo SS 3.8 Liter V6 Fuel injectronic, 4 spd elect. Auto trans, Sometimes it won't start, no crank, no click, no sound at all. The lights on dash work, head lights work, radio and interior lights work. After 15 to 30 mins. Car starts and works fine for a while. This has been going on for about 1 years 4 months. I have replaced the battery, starter and the MAF sensor 1 year ago. This has happen 4 or 5 times sense I have replaced that stuff. The Security light comes on about 2 or 3 times a mounth, and I get 16.8 mpg instead of 19 mpg. Every one I talk to said there is nothing can be done unless it happens at the shop so it can be tested.