I have a 99 Ford Explorer 4wd 4.0L EFI SOHC 6cyl. Car will start giving out on me when I'm coming to a stop and then idle erratically. As I'm taking off again it will hesitate and surge. Now this ONLY happens in the summer when it is fairly hot and usually after its been driven through out the day and at the hottest point of the day. Just seems like when gets to a certain air temp outside it will act this way. I am getting trouble code P0171 & P0174 or P0172 & P0175 can't remember. I changed the MAF sensor and checked the fuel pressure. I had an auto part salesman tell me they could hear a vacuum leak but only when the engine was revved really quick. I thought maybe the sound they was hearing was air being sucked into the intake manifold. I can't hear a one continuous leak and couldn't find one using propane and hose or carburetor cleaner. The car wasn't acting up when I was trying to find the leak or when the auto part salesman said he could hear it, would that make a difference when trying to find a leak? And am I on the right path looking for a leak or does it sound like something else?