Recently my 06 Monte Carlo started shifting funny. The other day, I was going along about 30-35, and tried to accelerate, it would not accelerate. The engine RPMs went up, but speed did not increase. It did this about twice before shifting quite hard. I've since noticed that only seems to do this when it has been running for a few minutes. When it is warm, it seems to be fine. I've scanned for codes, and there aren't any there. The fluid level is also where is should be, and looks OK. Is there something I should look at, or is it possibly on the way out?
Thanks in advance.

It is always best advised to drive it easy until the trans & engine are well warmed up.
Have you serviced the trans with new fluid & filter before? I noticed you are at 163k miles.
Once the pan is off, you can see if there is any clutch or particulate matter in pan and could consider installing a TransGo 4T65E accumulator shift kit while there as well.
If you do service transmission, treat it to some good Dexron VI (6).

When I started driving it, it was about halfway between cold and the normal temp it gets. When I tried to accelerate, I didn't his that gas that hard, it was a gradual increase in throttle. I did do a fluid and filter change back in the spring, and didn't see anything in the pan then. It just worried me a little because it has never done that before. I will probably drop the pan and see if I see anything.

When You changed the fluid, which Dexron fluid did you use?- hopefully Dex6 ??
Normally a P1811 code will show up, which scanner did you use?
Most of the time these transmissions experience hard shifts after being hot a while.
You may have varnish building up on solenoids or inside accumulator, so before you drop the pan and replace it with Dex6, add a half to a full can (depending how much room you got to just above full line when hot on dipstick)
Add SeaFoam Trans Tune.
Trans Tune - Sea Foam Sales Company
You may notice an improvement quickly, drive with it in for about 750 miles, after adding it, then change fluid again.

I did use Dex6 when I changed it before.
As far as the code scanner, I used one of those Bluetooth adapters and the Torque app on my phone.
The thing about it is, when it is all warmed up, it seems to shift fine.
I will probably give the SeaFoam a try and take a look at that shift kit.

torque only reads the standard codes not chassis codes unfortunately. when it is cold it doesnt lock till the trans gets to a certain temp, but since you said it hasent done it before i am going to assume thats not it.

Trans are P-codes, not B-codes. So they should come up on Torque Pro which is the same thing I use.

Torque pro is what I used, and no codes. There still are not any. I'm going to take it to a local shop that I've heard good thing about to see what they think.