need help to know what it is

[92stang gt 5.0]

okay when i am driving say in 5th gear and i am going 55-60 and i floor it the car starts to shake and stumble to go faster same thing happens in 3rd too.....is this something to worry about or will i need to replace something??

 

[stangbear427]

Only in 3rd and 5th? At what RPM? Is it a stocker, or have you changed some hard parts? Give us some backround here.