A/C Problem

I have a 2002 - 330 convertible. I am having an intermitent problem where the A/c blows warm sometimes. The dealer checked the refrigerant levels and of course the problem happens only once in a while. They said they need to see it when it happens. Left it at the dealer and they can't duplicate the problem. It is not a leak as they checked with a dye. Anyone else have this problem??