Why does my car have sporadic acceleration?

I have a 2003 Honda Element 4 spd auto with almost 140,000 miles and sometimes I noticed whenever I get on the gas from a complete stop, I have to accelerate faster than normally to get up to speed but occasionally if I give it a little gas, the car picks up speed quickly and the transmission downshifts... show more I have a 2003 Honda Element 4 spd auto with almost 140,000 miles and sometimes I noticed whenever I get on the gas from a complete stop, I have to accelerate faster than normally to get up to speed but occasionally if I give it a little gas, the car picks up speed quickly and the transmission downshifts responsively?

I experienced this whenever I driving on a COMPLETELY FLAT road with no small hills or any other steep grade.

Why does my car act like this? What could be the cause? The only thing I had a mechanic at pepboys tell me was that I need a 3 step fuel service cleaning, and after that bc I have a check engine light that reads a faulty o2 sensor I need a new set of spark plugs. This does not make sense bc I use iridium NGK plugs and they should last me for a long time. It was replaced around 95,000 miles. I changed the air filter. I’m suspecting the fuel filter might be a cause however my particular Honda model only sells the entire fuel pump system.
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