Hard starting when engine is warm only.

I would check your fuel pump to begin prognosis, be sure it is fully priming the fuel line and does not make any unusual sounds. Next listen how your flywheel on your starter spins as you turn the key to start your engine (is there hesitancy, grinding, etc?). From your description you mentioned starting, rough running once started, and smell of unburned fuel, after the engine has been warmed up; which is the identifying factor here. Valves are an important part in the distribution of fuel into the engine block, after the engine warms there maybe some blockage or expansion from heat. Be sure to check your distributor cap and other electrical connections, sometimes the slightest electrical sensor misread tells the vehicle it's not working correctly when it really is. I hope this helps or at least gets you started in the right direction

Try cleaning the MAF and O2 sensors. It seems like it's a flooded condition, when starting after a brief shutdown. Like the injector driver from the computer is holding the injectors open. The PCM can't tell if the engine is hot or cold. Cleaning the throttle body may help, as well as air and coolant temp sensors too.

What fixed the problem was replacing the fuel presure regulator.

You must check the alternator under full load as well as check the ground wire both where it connects to the battery and the transmission. Also check the coolant level. If it is alright then you must visit car service station. https://www.yourmechanic.com/cars/chevrolet

2000 k2500 7.4 L 454 . Starts hard after sitting.Starts great on fresh start. Starts great on having recent start. Put my scanner on it and fuel pressure gauge.Everything fine.No codes. One day while driving it quits running and needs a tow.I figured fuel pressure regulator and was right.It runs fine.Still starts the same. Put it back on scanner. Checked crank sensor and cam sensor because I got a crossfire code.Pulled out old crank sensor .It was carbon ed up,cleaned it. put in new Oriely auto parts one, No signal ,junk right out of box.I put old one back in its fine.Checked timing, distributor off maybe due to installing new cap. Runs fine ,same issue.Found out that its the temp.sensor in the block, no code,just not working right.What a pain it also controls fuel and air through the ECM.All these modern computerized vehicles are junk and I am sick of them.If you don't have the tools it will cost a arm and leg to get fixed. I'm going retro 1970's and back.