Why does my van lose almost all power after the first half tank of gas?

I had exactly this same problem and it was fixed by replacing the fuel pump. My theory was that when the pump overheated it's pumping efficiency was greatly reduced, starving the engine of fuel. The pump is inside the tank so when the tank was full the gas cooled the pump and worked perfectly. As the fuel level went down the cooling effect was less and the pump couldn't keep up. At less than a quarter tank the van would actually stall after running for 5 to 10 minutes. Let it cool down and you'd get another 10 minutes of driving. Fill 'er up and all was well down to a half tank. Several dealerships and garages missed this and I finally had to figure it out for myself.

Its not the pump but the fuel line from the pump to the bottom of the tank has started to crack allowing it to start sucking air into the fuel line. I had the exact same problem. But you might as well replace the fuel pump inside the gas tank while you have it taken off.