1998 Ford Explorer - unexplained problem

Hello,

My 1998 Ford Explorer Sport (2WD, automatic, V-6 SOHC engine) has stalled in traffic several times in the past two weeks. I am able to drive the vehcle 3-7 miles before the problem occurs. The vehicle starts from cold without trouble, initially shifts fine and everything seems to be okay. A few miles down the road, when I come to a stop, the vehicle stalls. I put it into park, turn the ignition and it starts okay. With the brake pedal pressed down, I engage any gear, and let off the brake just a little bit, and it immediately stalls again. It continues to stall after starting, when put into gear and brake pedal released. It is possible to "power brake" it, though, to move it off the road. Once the vehicle sits, turned off, for about an hour, it is possible to drive it a few miles before the problem happens again. There are no codes. (There was an 02 sensor code several months ago, that when cleared, never reappeared.) I have been to three mechanics, had it towed several times, spent over $500 in repairs (new PC valve, fuel pump and filter, and diagnoses), without resolution. Is it possible that something is heating up and malfunctioning that causes it to stall, but then cools down after an hour to allow the vehicle to be driven again? The only thing I've heard is that it probably needs a new transmission, but I want to check with you to see if there's a much less expensive repair to make first. It really seems like it's related to something that gets hot, causes a malfunction, then needs to cool off to run again. Any ideas for what that could be? I appreciate any help you can give me in how to proceed from here. I've had my Explorer for 10 years, have loved it, have regularly had it serviced, and have all of its maintenance records. I am currently on a student salary, so it would be great if I could find out what's going on and fix it, because right now I cannot afford to purchase a new or (unknown) used vehicle.