VW Volkwagen 2011 Jetta 4-door Sedan AC Air Conditioner Issues

I have a VW 2011 Jetta 4-door Sedan. Air always comes in through the vents when the AC is on, but the problem is that sometimes it is cold, sometimes it is not. It is unpredictable and intermittent, but in a 1-hr drive, definitely there will be AC and just air. What could the be issue or how can I troubleshoot?
Thank you.

My guess is a blend door is sticking.

That’s one indication of a faulty expansion valve at the evaporator.

Tester

A simple guess a part without diagnosis is a likely under $20 AC relay that is working intermittently. You can change it out yourself. I have self fixed my Subaru and Acura exhibiting same symptoms for $10/each with that guess.

This guess only works if you are referring to temperature of the air coming thru vents.