2002 Hyundai Accent Rough idle and car sputtering

I bought a used car about a year ago and it started giving me serious power delivery issues and some transmission problems. To be more detailed, the car would randomly get stuck in 2nd gear. Also, there would be very little power supplied by the engine; when trying to drive uphill, I would be pushing the pedal all the way to the floor, and the car would barely move forward if at all. I would have to throw it into low gear to move at all, and even that was slow. I changed the VSS, and now the transmission issue is not occurring anymore, but the car still sputters when accelerating from a stop, and the idle is still rough. The power is more or less normal now except for a the short sputtering on acceleration. The problem is more persistent when the fuel level is low in the car.

The car is also slowly consuming oil, (I'm not sure if that could be related to my problem). To let you know what I've already changed on the car: new spark plugs, wires, ignition coil, fuel pump, fuel filter, vehicle speed sensor (input and output), CKP sensor, and catalytic converter. No error codes appear upon scanning the ECU after changing these parts.