tranny slipping to neutral

hey all

i have a 99 with a automatic tranny and going down the road at maybe 55 mph it seems to slip into neutral .if you let off the gas it goes back into gear... and yes it does this quite often...i dont know what kind of fluid it has in it (read the other threads on the fluid probs) ...

anyway i really need some help on this one....

let me know what you think.....