I have a 1997 GMC Yukon with 160k on it. When I start the truck in the morning, I get a nasty hesitation that will almost throw you out of the driver seat. The hesitation continues while driving. I take my son 5 miles to school and leave the truck off for 10-15 minutes. When I drive it to work (after dropping my son off) the hesitation is nearly gone. If a smooth engine is 100% and a stalled engine is 0%, my engine's smoothness on the first drive is 25%. After the 10-15 minute off period at my son's school, the engine's smoothness is 80-95%. The engine remains smooth for the rest of the day (when I drive home after the truck sitting for 8 hours). Also, on the first drive (25% smoothness) the engine temp will work up to 200 degrees. Subsequent drives, the engine temp sits around 150 degrees. If I idle the truck for 15 minutes in the driveway and then turn it off for 5 minutes before I leave for my son's school, the engine smoothness is 50-80%. The following work has occurred before the hesitation started: rear freeze plugs, rear main seal, ignition module, pick-up coil, rotor, and cap. After the hesitation started, it has got a fuel filter, new plugs, and new plug wires, water pump, thermostat, and temperature sending unit (none of which has helped). The truck is throwing O2 sensor codes but it has been doing this for the past 5 years. Thanks in advance for your help.