1998 Chevy Suburban Cannot get it to start

For about 4 months my suburban was hard to start, then after making a 1800 mile trip to Oregon, it would not start at all. My Dad and I tested the plugs and wires and everything seemed to be okay. After putting it all back together it finally started and ran great. After running and getting warmed up it never has a problem starting. It is only in the morning or after sitting for several hours. We have replaced the plugs, wires, distributor, Throttle postition sensor, we have had the alterantor, and started tested along with the battery. Any other suggestions about what this might be?