1996 Oldsmobile Aurora Car sometimes doesn't start or even

I have a 1996 Olds Aurora that sometimes doesn't start or even turn over. At first I thought it might be the battery. Replaced the battery and the problem still happens. I have also so far replaced the starter, and even the ignition key switch. Sometimes, I can go whole month without any problem starting. When the problem does occur, I'll turn the key switch and not a sound. If I wait about an hour, it'll start right up. When the problem does occur, I have noticed that if I turn the keyswitch and just hold it in the starting position for about 5-8 seconds, all of a sudden it starts, but during those 5-8 seconds the engine doesn't even crank or turn over, complete silence and then boom it starts.
When I experience this starting problem, if I wait at least an hour or two, it will start right away. I have never had a situation where it didn't start at all as long as I let it sit for an hour or two. I have brought the car to my local GM dealer, and they haven't been able to diagnose what is causing this problem.