112k, 2.5 4 cyl, auto.
This started as an intermitent problem about 2 months ago, but is getting worse. When I accelerate to about 45 mph, the car will start kicking back. At first, I suspected an ignition problem, maybe a spark miss caused by a bad ignition module. Now I don't think that is the problem, because it feels harder than a spark miss. I have also suspected a transmittion problem, sort of like a hard downshift, but it does not downshift, but stays in 3rd gear. It also seems to happen too quickly for a transmission slip and catch. I am leaning toward thinking it is a fuel problem. The reason is that the error code says that the car is running rich - however this could be a another, separate problem. Anyway, I have a throttle body injection system, and I was going to start replacing some of these parts. My question is, which parts should I start with? I can replace any of the following: Idle Air Control Valve, Throttle Position Sensor, Fuel Injector and Pressure Regulator. Which is the most likely source of the problem. Or should I try something different? Thanks.

Could it be the torque converter locking and unlocking?

It could be. But that would make this car a "throw away". I paid $500 for it two years ago.