Does not downshift
I understand that it can be many different things, and here's what has been done so far. I've had the car for less than a year and I gave it a tune up shortly after I got it. The oil has been changed less than 1K miles ago. I replaced the fuel filter as stated before. I put a can of sea foam in when winter was coming on. Brakes were just redone 3 months ago, new pads and rotors. Transmission has not shown any signs of problems, and fluid level is fine. The air filter could use being replaced, but its not too bad (and I did blow it out with the compressor)
As for the throttle body/cable, I don't know their status. I'm still learning the car and don't know everything about it yet.
No, it does not want to downshift when I am at driving speed on the highway. It is an automatic transmission. I hope this helps a little. Thanks.