shifting hesitation

[jake1511]

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?