hello guys hope someone can shed some light lol

i have a fiesta 1.4tdci 2006

the air con is freezing as it should be but the thing is it comes on then after about 20secs turns off for about 20secs (the air goes warm) then comes back on it does it continusly while standing or driving,

any ideas??

also i have been told it could be the pressure switch, if so could i replace this without loosing any gas or will it need to be regassed, i know that if gas leaks i could face a fine this is why i am asking so i dont face this problem and get a air con enginieer to do it.

Thanks for any help