2000 Impala

My Impala has 111,000 miles on it. About three months ago I noticed that when accelerating the car, it seemed to "bang" into gear. I was told the transmission was slipping and to add transmission fluid to it. I was also told that the engine could be dirty so I had the cars engine "moto vaced" (thinking that the fuel injectors were dirty) and topped off the transmission fluid. This only slightly improved the cars performance. Lately the car has started to "bog" when reaching 60 to 65 mph, feeling as though it's "choking" for air. The air filter is clean. I also noticed that the fuel mileage this tank went from 379 miles per tank full to 290 mile per tank full. It doesn't matter what type of weather is outside or if the car is warmed up or not, it does it anytime.

Is this a camshaft sensor issue or another type of censor issue? I'd like to visit that before putting in a new transmission or some other major repair.