have an annoying problem with my 2001 4x4 Lariat Supercab F-150 with a 4R70W tranny.

putting it on Drive, if i slowly step on the accelerator, the truck goes fine. moves forward as expected.

but sometimes when i step on the accelerator a little faster, the RPM goes to around 1.9-2K, and it seems it didn't get into gears, then the truck jerks and moves on. i notice this when i come to a full stop or when i decelerate to around 10mph and step on the gas.

is there something wrong with my tranny? or do i just need to change the trans fluid now? it has 32,000 miles and i plan to change it on it's 35,000.

thanks for your help everyone!