Slow start after warm up.

From what you are describing, it is possible either the 3-4 actuator is no longer playing with the 3rd & 4th clutch packs, only way to know for certain is to hook up the good ol' pressure gauge to trans. system, yeah, you think the Chevrolet dealer should know this right?

Sell it now if you can!
Smiles

Leaking intake manifold (vacuum )

Have you checked the fuel pressure regulator to see if its leaking?

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

yikes. sounds like you've blown the head. $$$

NEED TO CHECK FUEL PRESSURE TO SEE what it is reading truck should have 60psi if it is lower it can cause symptons injectors wont fire right with low pressure

the problem you are discribing would lead me to belive your fuel filter is clogged which is a simple fix 3 dollar part at any part store or on the worse hand your fuel pump itself could be weak you can open the gas tank cover and have another person turn the truck key to the on position without going far enough to start you will hear a faint whining noise if it is going bad or a louder noise if it is fine Good Luck

I have a 1999 Chevy Silverado 5.3, It won't start. I drove it home, parked it, and the following morning it wouldn't start. On a couple of occasions, i have had a difficult time starting it. On both of those occasion, I waited about 30 minutes, then cranked it and it started with no issues following. I have replaced all the spark plugs, spark plug wires, fuel filter, fuel pressure regualtor, crank postion sensor, I have cleaned all the injectors, checked the injectors with a noid light. I have checked the fuel pressure what matched the specs in the repair manuel, checked all the fueses, i have a fully charged battery, what should i try next, taking it to a dealer? I have not replaced the fuel pump, that was the last thing i wanted to replace. any suggestions?

ICM idle control module, loc throttle body, it may be dirty, remove clean or replace, its easy..

hey if its turning over try shooting a little starting fluid in the air cleaner or a little fuel to see if it will start. if it does you got a fuel problem either pump of filter bad.. this is first thing i would do before tearing into something .. Jerry

I would have the computer scanned ( no SERVICE LIGHTS right ? ) looking for an errant coolant sensor not telling the computer to give your vehicle a bit RICHER ( a little bit more fuel ), since it does start. Just has POOR cold drive away. Hope this helps