from 27mhz to 40 mhz

[b4rs]

ok, many of you will probably know that i am currently looking to move from 27 mhz to 40mhz changing my transmitter. but can i just buy a 40 mhz transmitter or do i need to upgrade my servo and crystals to 40mhz aswell? i am a little lost on this topic and i dont know much about wavelenghts or even transmitters or decent makes. the reason why i want to upgrade is i am having problems when i use my buggy outdoors. it goes around 30ft away and then cuts out to 50% throttle even though i am pressing for full throttle. only when it comes into around 20ft away does it regain full speed without jittering. any help on this would be appreciated. do i need to upgrade to 40mhz? or do i have a different problem. thankyou
(sorry for the essay)