93 GMC Banging into Gear

hi, i have a 93 gmc sierra 4x4 5.7L pickup, recently it felt like the tranny was slipping, i know its not because the tachometer stays steady when it feels like this, it seems like it is very slowly shifting from
drive to over drive . when this happens i shift into drive an all is well again.
when i'm slowing down to almost a stop or driving slow in bumber to bumber traffic sometimes it will bang into a lower gear and actually stop the vehicle. scary!!
could the first problem be the torque converter? how bout the second?
Hope someone can help ThankTony
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Advice for Chevy owners, my uncle was a trans mechanic for forty years. He said if you change the filter and trance fluid between 17,000 and 20,000 miles and again at 70,000 the trance is designed to last 300,000.
These days, most of the dealerships & auto shops, have gone to the flushing of the trance; nothing like the changing of the filter and clean the pan and magnet. You can do it yourself, just remember to torch the pan bolts back to 9 inch lbs. Don't over tighten it might leak.

I'm lucky my dealership will still change the filter, but you have to request it.
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