1996 Chev won't start

I have a 1996 Chev 2500 Surburban 454 engine. When starting in the morning after sitting all night it will start on first contact of turn of key. After I drive around a bit and it sits for awhile it will not start until several off and on of the key. Fuel pump has been replaced twice thinking that was the problem. When put on a computer check everything checks out. No one can figure out why it won't start on turn of the key when warm and it will start on first turn of the key after sitting all night? Any answers?