Need help badly. 2913 Chevy exspress 2500

runs great when it starts. Some mornings you come into work and it will illy crank but won’t start

Every morning I come in to work I try to start it and it only cranks

this will go on for about a week. Then on about the 5th or 6th day, I’ll come into work, try to start it, and it fires right up. It will continue to start and run fine for 3 week, and out of nowhere . It sits overnight, you come into work, you try to start it and it only cranks

diagnostic code said ignition switch. We replaced the ignition switch, and it still does the same thing

any help would be appreciated