I experianced this very thing on my own 1990 Aerstar. I am a fair machanic and this one really drove me crazy.After 3 or 4 months and maney, maney checks, I found that someone had made a very neat repair to the fuel pump assembly. There is a little plastic bulb on the assembly after the actual pump but before the supply line exits the tank. This had been replaced with a piece of rubber fuel line. the rubber fuel line had a very small hole in it. The van ran very well on level ground and under 55 miles per hour. However when I needed max power the engine could not get enough fuel because the needed fuel was running out the hole in the rubber tube in the tank.