1992 Dodge Spirit Power Problem

I have a major problem and need some info. I got this car and it ran fine for 2 weeks. It has 117000 Km's on it. It would start to lose power as you're driving it so you have to give it more gas in order to get the speed back up. The speed and the rpm would both drop. You would also notice that even as you're driving the rpm's gauge was up and down and this problem was not occuring all the time, just sometimes. If it was acting up and you didn't give it gas, then it will drop to a certain level and then the car would get jerky. So you have to give it gas and it will take off but as soon as you level off then it would drop. At a stop, the rpm would sometimes be 500 rpm and somtimes 750. If you are stopping it would almost stall then pick itself back up again. I recently lost my transmission and had it replaced. I'm not sure if this is all related, but the car is still doing what it was doing before. I changed spark plugs, air filter. I'm getting advice from all over the place, possible fuel filter, or TPS(throttle position sensor), someone said MAP sensor. I have a huge hole in my muffler and had arguements about back pressure. My engine light doesn't come on but if you do the key switch back and forth you get a code about the oxygen sensor running too lean. Spark plugs does look like it's running lean. Any advice would help thanks.

Forgot to mention it is a 2.5 L automatic

if you do the key switch back and forth you get a code about the oxygen sensor running too lean. Spark plugs does look like it's running lean. Any advice would help thanks.

If the computer thinks it's runnning too lean it will keep increasing fuel to try and even out the mixture, are the plugs black and sooty? poor fuel mileage? I would start with replacing the oxygen sensor.

Other causes of this problem could be related to fuel delivery, a new fuel filter won't hurt, and then check out the pump and injector.