Has your car got an automatic transmission or manual transmission? If you have a manual transmission I wonder if the clutch has worn out? If so it will be accompanied by the engine racing up as you accelerate but then it will catch and transfer power from the engine to the gear box. An older car may also experience ignition distributor advance problems that would not "advance the spark" enough to make power but yet could maintain speed once it was achieved on the freeway. Fuel delivery problem (low fuel pump output) would usually result in fuel starvation, car may accelerate to a point and then loose power and momentum under sustained load.