Ford :: 1996 Ranger Idles Roughly, Stalls Out During Hot Weather

I recently purchased a 1996 Ford Ranger with the 4 cyl. engine. The first 10 days I owned it it ran just fine. I then took a an hour drive to another city. After the first few stops there, as the temperature outside reached about 90, It started to sputter and idle very badly. After I got rolling it would would run fine. I decided to try to make it home. It ran fine at freeway speeds of about 70 mph. I had to stop once; it stalled out and would not start again. After waiting about ten minutes I tried again. It was still running rough, but I was able to get rolling and drove the rest of the way home without any problems until I was in town and had difficulty keeping it running at idle.

The next morning it ran fine again. A local mechanic changed the fuel filter, but the problem came back as soon as the temp neared 90 degrees. A mechanic friend who witnessed the problem said that it definitely sounded like a lack of fuel and not an electrical problem. Is there a way to troubleshoot without swapping parts? If the fuel pump is failing in warm weather, how is it possible that it runs fine at 70 mph, but not at idle? Engine temp gauge looks good. I did get one code after it died the first time (lean mixture, I think he said).