if you have an auto, it is the transmission. maybe it is downshifting right when you floor it, and then upshifting once you get up into the 5k range. i think that autos do that so that it is easier to pass on the freeway and stuff. you can accelerate better in a lower gear, which helps get around other cars. maybe that is the cause?
im not sure.
if you have a manual, then i dunno what is going on. maybe your car is possesed