98 Ford Explorer Eddie Bauer-Auto transmission shift problems

Hi Y'all,
I have a 98 Ford Explorer 4x4, Eddie Bauer SUV. It's an auto
transmission, and for the past several months it intermittantly will
not upshift when it should. The engine will rev really high then it
will chunk into the next highest gear. I have recently had the
transmission fluid changed and the serp belt changed too. But it still
does it. At this juncture if it does it I will lift off the accelorator
in order to let it 'manually' shift itself. Any ideas on the why? or
how to fix?
Any ideas or information would be greatly appreciated.
Thanks,
Angie