I'd start poking around the tank venting system.  Check the hosing, and
look at the condition of the charcoal canister.
--scott

   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