1996 Nissan Pathfinder 1996 Nissan Pathfinder Engine stalli

In the last couple of months I have noticed my engine power has dramatically decreased. Sometimes when I press on the gas pedal I feel like nothing happens, and I lose a lot of engine power when going even the slightest bit uphill. Also sometimes during operation the car will stall, most often immediately after startup and putting the car into gear and hitting the gas. Other times it can take me several tries to get the car even started. I've taken this to the dealership, as well as several different mechanics and no one can give me a clear reason why my car is acting this way. One mechanic will tell me what another mechanic said is wrong so on. As of yet I have been told it could be any of the following: an exhaust restriction, a bad fuel pump or injector/s, something in the throttle chamber, a bad distributor cap, and one or more bad pistons. Now recently I had some of the spark plugs replaced, but I don't think all were, and I'm not sure if the wires were replaced. Also the O2 sensor was replaced at that time. Any help anyone could offer me would be greatly appreciated, thank you.