1988 Chevrolet Impala

I have a 1994 Delta 88 with an intermittant problem. When I am going in a clockwise circle- entering or leaving the expressway- or sometimes going over a bridge or, rarely, in a straight line, the gas gauge needle heads to zero and the car acts like it is out of gas. Several times it has even died as I am about to enter traffic. I shift to neutral and it starts after a few tries (fortunately I haven't been creamed by a semi). This generally happens when I have half a tank or less of gas and the car has warmed up. My fuel guage is off by an eighth of a tank (ran out of gas at one eighth of a tank when this trouble started).
Of course this didn't happen at my mechanic's, so he finally drove it around for a week until it acted up and felt like a fuel sensor problem. Replaced the sensor. A few days later, when it was dying every few minutes on the way home, I brought it back. He checked the fuel pump, even removed the gas tank and checked whatever is in there. Couldn't find anything (didn't charge me either-nice guy) but it no longer stalls on the straightaway and if I keep the tank full it behaves. He checked with other mechanics and found another 94 Delta-88 with the same undiagnosed, weird problem. He said it would be too expensive to track down the problem, but I worry if I can't get to a gas station often enough. Any ideas besides poultergiests or aliens?
Oh, also, when I step on the gas pedal, sometimes it feels like it's pushing back. (Twilight zone theme, here)
January 3, 2006.

Thanks. I'll try that.

Jan 7, 2006.