lower acceleretion and hesitation sentra 1985

I did read through this forum and couple of others in the internet. Did posting here and there. I read through couple of thread especially Torch who might have the same problem as I do in this forum. My car is manual trans(b11, e16s), carburated and have a mechanical fuel pump. It idles at 800-900 rpm at normal operation and it never stall or die while it is idling inside a garage or traffic light. I did check the ignition timing and its at 10btdc. Check the timing belt and it is tight and snuggy. Lately its been losing power after driving for 15-20 minutes. When it loses power after driving for 15 to 20 minutes, the rpm moves erratically between 2500 and 3500 rpm. I have to rev it up to 3000 or 4000 rpm and keep on tapping on the acceleration pedal and to move by pressing the gas pedal all the way down to the floor and I have to press the clutch pedal to half way to keep it from dieing on me. It makes a huge revving noise when I try to accelerate but does not have the power to accelerate and moves very slowly. But it gets me home. It used to be like 30 or 40 minutes of driving before it starts to lose power but nowadays its about 15 to 20 minutes. Something is slowly getting from bad to worse. I did check and replace new vacuum hoses where it is needed. I made sure all the groundings are clean and tight(unlesss i missed something). I did changed the spark plugs, spark plug wires, fuel filter, coil wires, rotor, changed the vacuum hoses, and changed the ignition coil. I did change the old ignition coil with accel performance coil, but it did not solve the acceleration problem. I did used the carb cleaner to clean the carb and done it couple of times. I also clean the distributor by opening the distributor cap and blowing dust out with electronic contact cleaner and compressed air. Its dry and clean inside the distributor. I also made sure the connection point are clean and tight at the ignition coil bracket. My battery has 12.4 volt without running (key at on position at ignition switch) and 14.3 volt when its idleing. I made sure the connection are clean and tight on my battery too. The battery and alternator test out good at autozone. I ran the car until it run out of gas and fill it with some chevron gas, still no luck. All these changes did not fix the problem. Thank you in advance for all your suggestions and ideas.