2009 Silverado 1500 4 X 4 LT 4.8L Transmission issue?

...So I did research, even brought my truck into a proper mechanic who confirmed, and could have sworn my 4 speed auto transmission was going. Original problem was the engine light went out after a few hours of driving a week ago. While the 2nd gear starts to shift into 3rd it would cause the RPM to rev very high...only remedy was to release the gas pedal and apply softer pressure. Not to mention it jerks very hard in between gears. RPM at 65mph sticks at 45OO.
Just filled up gas tank and now no issue!!
Was my transmission misdiagnosed? Could it be something else? I Even bought a re manufactured tranny and waiting to remove/replace come Thursday...
Any help or 2cents is cool.thanks

Oddly that sounds like what happened to my '03 chev with the 4.8l. Although I didn't have any check engine light. It only happened once, I could of swore the trans was going, it ended up that when I put it into drive the shifter actually moved in between the gears between 3rd and Overdrive. I didn't realize it because my shift display in the dash didn't work, and until I bumped the shifter and it moved up the slightest little bit and every thing worked fine. I am a mechanic have experience trans problems before, but not like this, talked to a buddy that has been in the trade a lot longer than me and he said since my shifter moved, it was most likely more than one valve opened or closed more that it should in the trans, and that it cleared up when the shifter was move. it has been 2 years with no issues. How many times has yours acted up, or was it only the once?

the transmission requires a pressure test procedure . this will reveal if its a electrical or internal tranny pressure issue.. when the techs service these they bend the shifter bracket at the pan . this can throw off the shift adjustment or the adjuster did get out of proper position. then the intermittent does point to an electrical this can be the tranny electrical connector/wiring . on much older GM trucks 1990's the ignition switch caused these issues which drove many nuts... the forums have helped these owners.. on a 2009 this is probably not the ignition switch but ???