AC works for 20 min or more then blows hot air. turn off and comes back

[Blake]

My AC has been serviced two times since I have had it. And it work fine. But it will suddenly lose its temp and go hot. I can turn it off and back on and it comes back. I am thinking the thermostat but am not sure. Has anyone had this problem. And my exp has the old dial type controls not the electric FYI
Even when I turn on the heat it wont change.
Is this in the control or is there another part near the AC?

[Blake]

now I have learned that it could be a vacuum line.
Any help would be great.

[Cherokee]

could be low 134 level