99 Silverado help

Hey everyone, I have a 99 Silverado Z71 that has been acting interesting lately. It has 120,000 miles on it so i'm kinda beginning to worry a bit. At times, when
driving it will idle about 750-1000 RPMs and you can feel it pulling when you come to a stop. anyway, when you accelerate, it feels like the transmission is holding the gears long and the shifts are harder. The 'tow' option is not engaged and in just regular overdrive. I find that it does this on occasion and i've had the transmission serviced/flushed at 38,000 miles and 75,000 miles by the Chevy dealer. I also sometimes find that it does this right after taking it to the dealer and getting the fluids changed. most of the time when i cut it off and come back and start it back up things are fine. Are there any thoughts or suggestions of what it might be and is this a precursor to things going wrong? Thanks in advance!
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First thing is first, head to your nearest Autozone and have the PCM scanned for trouble codes.
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