car dies,stalls or hesitates when fueling

Weird problem. To me anyway. Just purchased a 94 Grand Prix 3.1 litre 2 weeks ago. When I fill up with a tank of gas, whenever the fuel guage is over half the car acts like the problems in the subject line. Half of tank and under it doesn't do this and runs like a new car. Seems like the more gas in it the worse it runs. I know so don't fill it over half then. But anyone have any expertise at seeing this problem?

Look carefully & see if bottom of tank has hit something like a curb etc. Some GM tanks have a module(not sending unit/elec. pump) affixed permanently to the inside bottom of the plastic/fiberglas-feeling tank--there will be a cast indention, circular most likely, in underside of tank under where it sits. If tank has been bumped, this module can come loose and do fine with well under a full tank. Had an '02 LeSabre with that problem & probably identical symptoms. I bought it from a GM-auction who had bought it back under the Lemon Law. Had been to 3 dealerships to be fixed; turned out all 3 worked on wrong end. I took it to my favorite mechanic at a Buick D'ship & he found problem in 10 mins--GM's loss ="d my gain. They replaced tank under warr (was only few mos. old w/6000 miles) & cured problem. Drove it to 30K & sold to friend. Still doing quite well today. Good luck in finding your problem. s