Hi Jim.
I had the same issue, I bought the car in December last year so didn't notice anything until we had a warm day in March when it didn't cool down at all. I went back to the garage where I bought it from which is about 110 miles away, so ten gallons of petrol there and back, seemed okay for about a week to ten days then stopped cooling again. I then got a local chap who did the vacuum test refilled the gas it worked fine for another ten days then stopped. So as Joe said I called in an expert who charged £30 for an inspection when he found the fault which was in fact a tiny hole in the condenser. He ordered one and replaced the defective part, it did cost £360 but well worth it as we were leaving the same week to drive to Sorrento. Needless to say the garage I bought from declined to even contribute to the repair, bad mistake as I won't go back when it's time to change the car. The car ran well all the way down and back covering over 3000 miles averaging 26.5 mpg.
Bob