F250 air con blows cold then hot
 

We have a 2003 F250, 4.2 litre, 130000km,

Our air con has been regassed twice, the mechanics did this because they didn't have any other answer, the vehicle has been hooked up to ford computer where no faults came up, showed all systems are operating normally. All pressure in air con is good.
Still, the air con blows cold for 10min and then hot I turn it off for a while and when I turn it back on it blows cold for 10 min and then hot.
We would appreciate any help with this matter

Cheers Flossy :(