97 7.3L deisel starter problem

now and then my starter spins but the bentex does not engage, the problem came out of nowhere. I have since changed the starter twice with the same results from the three different starters. i have checked my connections and the grounds, the truck always starts after two or three times, but sometimes will go weeks without a problem at all. i have also checked both batteries, and recently changed the flywheel when my transmission was rebuilt