Does it do this with a full tank of fuel? Check on exactly when it does this, the more details the easier it is to track down. If it does it only when the car has less than half a tank and it gets worse the lower amount of fuel you have in the pre pump (in tank fuel pump) is my suspiscion. These little pumps fail and really are only there to pull the fuel from the bottom of the tank, when the tank is full the fuel itself (with gravity's help) pushes it out enough for the car to work fine. Also check to make sure there are no leaks in the sending unti coming from the fuel tank. A pin prick hole could cause these symtpons as well. The metal lines going into the unti rust and unless you tap on them you may not be able to find the leak. Check to see if there's any rust on them. In the trunk (or hatch of a wagon) under the carpet on the drivers side there is a small metal panel with a few screws in it take these out and you will be able to see the sending unit. Use a sealed light as if there is a leak there will be gas fumes in that area. You can also check by pulling the fuse for the pre pump and see if this makes a difference, if there is no difference in how the car runs with the fuse out and it only does it when there's less fuel in the car it could be your pre pump. Soemtimes the strainer on the pick up tube i the gas tank also gets clogged and this could cause it as well but it would more likely happen all the time.