1995 Mitsubishi Galant hesitation

My Galant hesitates or bogs anytime I press into gas 1/3 of the way down. If I accellerate very smoothly it wont bog. It will hesitate cold or warm, seems worse after car is warm and really bad if I am on an incline or needing to accellerate quickly, or just leaving a stop light and stepping on gas slightly too hard. Feathering the gas does not help. I have a check engine light on and the code pulled said rear O2 sensor bad, I have not chaged it yet. I have changed spark plugs in last 5000 miles, fuel filter twice in that time, and have added 4 cans of fuel injector cleaner to the gas tank. I drive 20 miles to work everyday at highway speeds and have no trouble when car is at those speeds, but accellerting is smooth then, even on hills in cruise control. The car starts great and idles fine.