Highspeed Acceleration Misfire/Chugging

[95BuickRiv]

I had purchased gas from a cheaper gas station and felt misfiring 5 miles later. Everytime I am on the highway and accelerating above 50 mph my car chuggs and has pretty much no power or acceleration. I can kick it down anywhere between 0 - 50 mph and its fast and runs fine, also idles fine. I have changed the fuel filter (twice), oxygen sensor,spark plugs,PCV,pulled the fuel pump out and checked the gas tank for rust etc.,cleaned the pump strainer, put NOS octane booster, fuel system cleaners, and other injector cleaners. But the problem is still there but is better than the day it started. If I blow out the filter some small particles of rust come out but not everytime. When I blow it out and put it back on, take it for a drive it runs good, even a high speeds.....but as soon as i shut my car off and let it sit the problem comes back. Does anyone know what the problem may be? Its been 3 months and its driving me nuts! can pumps start to fail in this way or do they just usually stop working?? Please Help!