a/c need some "cooling time" after about 1 to 2 hours driving. why? on 2001 BMW X5

after 1 to 2 hours driving, we could hear the blower still on, but there was no wind, no cold air coming out of front outlets, but it was nomal for back seat outlet which we kept it on about one hour ,then the a/c (the front outlets)got back to normal. We had a new "the final stage blower motor resistor" in bmw dealer, the problem remains.

Asked by for the 2001 BMW X5