No mechanic has been able to fix!!!!

I have a 1999 Chevy Silverado 5.3 V8 Z71 with 117,000. After sitting overnight when you start it it idles down kind of rough like it might die. If you put it into gear it almost dies and when you hit the gas it has a slight 2-3 second delay. For the first 5-10 minute drive time, iwhen you come to a stop, it will idle up and down, up and down. After that it runs fine all day. If it is at night, the lights go dim and bright with the up and down idle. If after starting, you let it idle for a good 10 minutes it will take off fine.

This is everything the Chevy place has done. New fuel filter, PCV valve, new sparkplugs and wires, new battery, new alternator, cleaned throttle body, has had all software updates for the puter', new fuel pressure regulator, and nothing has fixed it. Does anyone have any ideas? I am running out money to keep having to replace stuff that might not have even needed to be replaced. Help me please!!!!

I have a 1999 Chevy Silverado 5.3 V8 Z71 with 117,000. After sitting overnight when you start it it idles down kind of rough like it might die. If you put it into gear it almost dies and when you hit the gas it has a slight 2-3 second delay. For the first 5-10 minute drive time, iwhen you come to a stop, it will idle up and down, up and down. After that it runs fine all day. If it is at night, the lights go dim and bright with the up and down idle. If after starting, you let it idle for a good 10 minutes it will take off fine.

This is everything the Chevy place has done. New fuel filter, PCV valve, new sparkplugs and wires, new battery, new alternator, cleaned throttle body, has had all software updates for the puter', new fuel pressure regulator, and nothing has fixed it. Does anyone have any ideas? I am running out money to keep having to replace stuff that might not have even needed to be replaced. Help me please!!!!
This is a temperature issue. On an older car it would be the automatic choke which keeps the rev's up while the engine is cold. On new fuel injected cars as yours, sensors are used. Did they not check the codes ? I would be leaning towards a bad "AIS automatic idle speed motor" ... I hope this helps
Sam

I've had a bad tps cause my engine to idle erratically as well. Another thing to look for is a vacuum leak since once it heats up things are fine.

I have a 1999 Chevy Silverado 5.3 V8 Z71 with 117,000. After sitting overnight when you start it it idles down kind of rough like it might die. If you put it into gear it almost dies and when you hit the gas it has a slight 2-3 second delay. For the first 5-10 minute drive time, iwhen you come to a stop, it will idle up and down, up and down. After that it runs fine all day. If it is at night, the lights go dim and bright with the up and down idle. If after starting, you let it idle for a good 10 minutes it will take off fine.

This is everything the Chevy place has done. New fuel filter, PCV valve, new sparkplugs and wires, new battery, new alternator, cleaned throttle body, has had all software updates for the puter', new fuel pressure regulator, and nothing has fixed it. Does anyone have any ideas? I am running out money to keep having to replace stuff that might not have even needed to be replaced. Help me please!!!!

I would look toward the sensor readings during a cold start. A coolant sensor reading a few degrees different than ambient air temp (after sitting all night) will effect cold driveabiltiy.
Also what octane gas are you using. Anything more than 87 octane can cause some undesireable cold driveability caracteristics.
If the wrong calibration was installed, there are some driveability concerns that can occur (some calibrations are for specific problems..if the PCM has one of these calibrations installed this could cause a problem)
Fuel pressure could be on the low side of specs or just under spec..this can cause a cold problem.
If you have a cable operated T/B, there was a TPS spec that needed to be adjusted..if not, this could also cause a problem