Trailblazer transmission issue

[Swats]

My mom owns a 2008 2wd Chevrolet Trailblazer. Inline six cylinder with a four speed automatic transmission. It has a little over 17,000 miles, but occasionally tows a 24 foot travel trailer. Last night dad took the truck out to put gas in it for mom. On his way back dad was going about 45 mph, and coming up to a red light. He left off the throttle, and felt there a was a notable amount of drag on the vehicle. Dad described as being like that brake pressure was being applied, but he was not on the brakes. He applied a little throttle, the rpm's came up but the truck did not accelerate for a few seconds. I told him to take it to a dealer for them to figure it out. On the way to the dealer this morning. Dad was stopped at a light. He accelerated away from the light and was going about 45 mph. When the transmission started to shift back and forth between third and fourth gear. He said it did that for a few seconds. The dealer techs could not get it to act up for them. They did acknowledge that the trailblazers do have some transmission issues. I just took the truck for a second test drive and it did not act up for me. Any ideas? Thank you for your time.

 

[Detroit Gearbox]

If the vehicle has no dtc's, and it no longer acts up, my guess would be a input to the ecm, such as mlps sensor, speed sensor, tps....something either is giving the ecm the wrong information or the ecm its self is bad. There are many posibilitys.