Q: Rough Idle and Stalling at Operating Temp

Hello,

[2002 Nissan Xterra SC] Car came with a few codes on it. One being the knock sensor. Had a mechanic replace the knock sensor and all the codes of disappeared. Since I have driven it quite a bit to ensure that no more codes come or that the "check engine light" comes either. There are none.

3 issues happen when at operating temp, no issues when cold.

1) Bad Gas Mileage. Drove about 135 miles at 75mph and got 7mpg. Drove same distance back at 50mph and got 11mpg. 2) At low speeds(0-15), coming to a stop or at idle; it has issues with sputtering like it is about to stall and die. It will stall and die about 50-75% of the time if I don't try and give it gas, although sometimes trying to give it gas doesn't even work. 3) Higher speeds it intermittently will have an issue speeding up or upshifting, tends to need me to let off the gas to upshift in order to go fast. Otherwise it will hold at 4-5k rpm unless I let off peddle. Randomly will lose power and I go from 75mph to 45

My car has 175000 miles.
My car has an automatic transmission.