A/C blows cool for a while then blows slightly.

A/C blows cool for a while then blows slightly cool or at outside air temperature. Eventually it will go back to blowing cool again. I took it to a mechanic that said it was the A/C compressor relay. He was not able to find that Infiniti part without buying the whole $800 module. He did find one from a Nissan that he said looked identical and installed it. The problem seemed to go away for about a week and is now back. Does this part actually sound like the problem? If so, should I try to find one used? If not, what other suggestions do you have.

Barry