1997 Ford Aerostar lean condition

I get a check engine light about 3 miles after I leave my house in the morning. The code was lean on right bank. I replaced o2 sensors, but did the same thing again, at about 3 miles from my house. I borrowed an analizer and reset the code as soon as it happened again. Both times the computer showed 156 degrees temperature at about 1700 rpm. After resetting the code, I continued driving, bringing the engine up to normal operating temperature. I continued driving without any light coming on. The rest of the day was uneventful. The outside air temperature has been about 90 degrees. The van really never had a chance to cool completely down for the rest of the day. Tomorrow morning I expect the same thing will happen at about the same place on the hiway on the way to work. Air cleaner is clean. Mass airflow sensor was replaced about 2 years ago. The car does not lack in power at all. It idles very smooth. Help. Thanks, Jan Hamud