2005 Yukon Denali Transmission issue

My Denali has the 6.0, I believe that is the 4L65E. Truck has 120,000 miles. Seems like there is a lag and possible slip between shifts when driving normal. seems better when I pour the coal to it. Wondering if anyone has a thought on this. My guess is maybe the PWM solenoid or maybe even a bad clutch pack. Are any of the transmission additives of any value whatsoever or are they just motor honey?
Thanks for any thoughts
Bill

The recommended "transmission additive" is to remove the pan and filter, "add" a new filter and "add" 5-7 quarts of clean Dexron VI (never Dex 3) and hope it cleans up the hydraulics. If you find excessive metal or piece of friction material in the pan when it is removed, you'll know to plan for a rebuild sometime soon.

Mrs. Cusser has that exact same Denali. The transmission went out in April 2016 at 165.7K miles, towing the horse trailer and horse. She was able to continue driving using 2nd gear, then we had it towed and the transmission rebuilt locally at a transmission shop, was $2.0K including labor, etc., with 2-year warranty. It has worked fine since then.

Thanks for the input guys, I have been on the road since I wrote this post. I bought the trans filter and fluid and was set to change it, but ended up in Texas, Tennessee and then California. Transmission cast it's withers last week before I had the opportunity to change the fluid and filter. Hoping to to get the call today to pick it up at the Transmission Shop.
Hopefully it will last another 130k