1994 Oldsmobile 88 Engine problems

Hello,

My engine lopes/hesistates at high RPMs. Runs fine at idle and low rpms. The longer i drive it the worse it gets, it will start to lope at low rpms after a while and eventually cut off all together. This is after like 45 mins of driving . If i have less than half a tank o f gas then it takes about 20-30 mins a full tank it will run longer. Ive dumped a ton of money into this trying to figure out what the problem is. Ive replaced the spark plugs, wires, coils, ignition module, egr valve, air filter, fuel filter, fuel pressure regulator. I recently took it to the dealership and had them diagnose it. They told me that the engine wasnt getting enough gas caused by a failed fuel pump. I got that replaced along with the strainer and fuel filter. After testing the car out it still did the same thing. I unhooked the fuel line before the fuel filter to see if it was pumping and it was. I checked the pressure at the engine and it was where it should be. So now im at a stalemate. Do you guys have any idea what the problem could be?