Ford Repair: no power, low gas milage, fuel pressure regulator, egr valve

Question
I have a 94 ranger 2.3L 4 cyl 2 whl drive with about 120K. for a while now i have been getting really bad gas milage about 10 miles to the gallon. the check engine light goes on and off but never stays on. i took it to an auto repair place and they told be the O2 sensor was bad. i replaced the part and the power was good for about a day. now its back to its weak power. Driving up hill is a strong struggle and if its steep enough it wont make it. What do you think is the problem with it? fuel pressure regulator, catylitic converter, or EGR valve?????? or all of the above???

Answer
I need some more information please.I need to know what engine codes are present.Did you only replace one sensor?Which one.Probably need to replace the rest of the O2 sensors.Maybe none of you're above.The converters can be tested with an infrared thermometer.