This is expected and should be remotely proportional to how long you are away from it going about your business.

Sounds like a flare.

Is the transmission fluid full?

How many miles on the transmission fluid and filter?

Could be a few things causing a rough idle and/or shift flares.

Also, what type of air filter is it?

Usually, unless the PCM is fighting some other issue(*) at the same time.

Fluid and filter are at 100k, fluid is full and still the normal color, slightest little bit of brown but I am doing that when I get the fluid and filter and pan gasket in on the 25th

The air filter is a cylinder type filter that sits in the filter housing, just oem.

For the coolant temps I am aware they drop, just was saying that when I turn the car back on with it only mildly warm, I get that shift hiccup on the first accel.

That'* a large generalization. I haven't heard this in general and haven't seen it on a properly running LS. I have a 2001 5.3 LS that is a distant relative of your 6.0 that has almost 320,000 miles on it and it is perfectly smooth. Biggest issue is some lifter noise, but still it is perfectly smooth at all speeds including idle.

For what it'* worth, all of the OHV Chevy V-8'* I've run across over the years, big or small, run pretty smoothly when they are stock, tuned well, and not fighting a plugged catalytic converter or air filter. I'm not saying excessively tuned beyond factory specs, just tuned to factory specs. I'd happily put a glass of wine on top of any of them. Would there be a slight ripple? Probably. Would it spill? No. Do I feel anything inside while they are idling? No.

Put in a cam with a lope, an aftermarket tune, or have a bad injector or other maladies, and they can run rough.

Cool! This might help with the flare. Some of the 6-speeds had some issues with this, even when new.

Okay good. I know that some are fans of oiled reusable filters like from K&N. I am not, and have seen many bad results from these filters, usually including gummed up sensors etc. Glad that isn't likely part of the problem here.

Gotcha. Yeah, some of the 6-speeds do that a little, but the filter and fluid change might help with it, especially if the flare behavior goes away after it is completely warmed up.

Ya this is completely stock. Looking at live o2 data it seems pretty normal, b1s1 and b2s1 (both upstream) fluctuate rich to lean like they should, downstream both sit at 650/700 mV. I dont think its a cat. I just have 0 clue why my trims are fine but I have this little shake at idle. Its annoying. Literally some days it will run like a perfect motor at idle for a few seconds, then just a tiny disruption every couple seconds, other days its just like dang what is happening in there. RPM only fluctuates from 580-590 so not that much at all and I cant find any data in the scan tool that shows something off

ya its literally just on the first accel when its still warm but coolant (and trans fluid temps) are not at operating temp. Just the first 1-2

Also if it means anything, the ecm doesnt have any misfire'* detected, or any history misfires... so idk if chevy only "detects" a mis if its a dead mis or what

Do u think its weird that the throttle is only open like 3% at warm idle? I am used to seeing at least 6% on the german cars even with big motors.

I think this has to be related to ignition or a dirty sensor providing false data. Map sensor is reading 4.3psi at idle which is a little lower than expected and maf reads 6.3 at idle which is a little higher than I would expect

Sorry last question... is it normal on these v8'* for timing to be at 40 degrees when accelerating gently? And then 50 degrees when cruising?

I mixed up something. It'* not IPW that correlates with displacement it'* air flow. A 6L engine will have a MAF reading of about 6 grams/sec at idle.