One area not yet explored may be a defective gas tank. Our 98 Mystique used to get that way on 1/8 of a tank or less. It was expained to me that the tank was defective and did not vent properly. The symptom was what you describe. The cause was that the tank not venting properly becam concave in the bottom and registered far more fuel than was in the tank essentially causing fuel starvation and an overly optimistic gas guage. It may be with a look see! P.S. the solution was a new gas tank!
John
