Car losing power in 3rd and 4th gear

[m.anctil]

Hello,

I am just having this problem with my car, recently I have noticed it has been losing power in the 3rd and 4th gear. When I accelerate from a stop the first and second gear are fine, but as soon as I hit 3rd I notice it kind of lags, and then mpg meter goes down to only 20. Then when I go to 4th theres even slower acceleration and the meter goes down to 15 mpg. Any help would be great, its an automatic btw.

[96_323]

Cant be sure but id say check the M.A.F.
Whats it like in reverse?
Have you are anyone else been tinkering around with the car recently like doing any mods?