what you're describing sounds to me like 'flash stall', which is a torque converter function..
flash stall is a measurement all by itself that a lot of folks don't count on.. the basis (if I can explain it decently) is this:
an automatic transmission is driven by hydraulics.. fluid moving, swirling the direction the engines crank is turning it.. the torque converter is a fluid coupler, which is physically attached to the main crank of the engine and forces the fluid 'attached' to the transmissions main shaft to swirl the same direction..
if you're idling @1000 RPM, and absolutely stand on the throttle, NOTHING will happen until you reach 1500 RPM... this would represent a 'flash stall' of 500 RPM.. it sounds just like slip, and feels like it for the best part, the only distinguishing factor is when you're slipping, the truck will actually attempt to move.. below and during flash stall, it won't..
think of it like this:
you have a bucket of water in front of you, and a paddle in your hand, resting in the bucket.. you're about to start vigorously stirring that water, like you're mashing the sour for a good stihl run..
instead of starting out slow like, you start stirring like a mad man- and it is a second or two before that water starts uniformly flowing in a circular pattern....
the time between when you start vigorously stirring and when the mash starts uniformly swirling is that bucket of Sour Mash's 'flash stall' rate...
the flash stall has a lot to do with the volume of the transmissions pump, the volume of the torque converter, and the quality of the t/c's stator function.. if you notice your flash stall starting to lengthen a considerable amount, you may need to start paying close attention to the t/c and transmissions health.. it is a good indicator things are wearing out- specifically the stator (which is the gadget that allows that swirl to reverse direction on the same plane)..
transmissions are floggin' intricate and intimidating to figure... but their function is pure simplicity.. it's all about the transfer of fluid, which can dang near be explained every time with that bucket and how the water responds...
edited to add: a high flash stall isn't a bad thing... think of it as getting a running go at something.. if you've built up a lot of inertia, your first obstacle is less likely to stop (stall) you dead in your tracks.. too much, though, and you burn a lot of fuel and put some needless high temperatures in your transmission/ and t/c..