It may be that the refrigerant level is slightly below spec. I had this problem last year when I was travelling through Las Vegas.

I tried one of the do-it-yourself kits and though I followed the directions didn't think I over-filled, I found that it became slightly worse at times!

I was told on return home that the A/C system can be very finicky with refrigerant levels. I went to an A/C shop and had the System flushed and recharged and problem solved!