ok this one is confusing me.

my gf's 99 automatic cavalier with 114k miles on it shifts funny when she opens the throttle wide. it does it sometimes when its idle and most of the time when shes coasting and then hits the gas hard again.

after she hits the gas pretty hard it revs a little bit like its in neutral and then shifts. i cant exactly remember what gear it does it in but i know it does it often.(she says her cars showing off :P)

what causes this and is it expensive to fix?

 

I found your problem, actually 4 of them

firstly, Youre GF is a girl I take it? chances are some percentage of it is her fauilt.

Secondly, Its an auto

Thirdly, Its a 99 cavalier

And last but not least, It has 114K meaning its well past its expiration date.

 

gm made a feature in their cars to cause the hesitation. so if you stomp on the gas, it will not engage right away (but this includes revving up). this causes a momentary hesitation, so the car does ruin the tranny.

if she tries to take off and floors it, and it takes a little while to shift down, and just to plain out shift, then you should probably check the tranny. it could be as simple as a clogged tranny filter (i assume she has an automatic). but if that doesn't fix it, you might just want to get another tranny or get rid of the car