I've been doing a lot of work on my impala due to several strange issues, I replaced shift solenoid 1-2 because it was fried and threw a DTC. But outside of that the car has been behaving unlike anything I can find on the internet. When I take off the car feels incredibly underpowered in first gear, but as it gains a little speed the engine kicks way up in rpm, similar to how it feels if you turn on cruise control and hold accel, and then it kicks down again, it does this about 2 or 3 times before it shifts gears and kicks down at 55. It's definitely not slipping, because it speeds up relative to the rpms. No check engine lights and no DTCs, so I plugged the code reader in and noticed that the long term fuel trim is excessively low, sitting around -14 to -21. I replaced the MAF, and the TPS, but neither of them made the slightest difference. It still runs incredibly rich and nothing online has been remotely helpful. It's worth mentioning that it's been doing this since before I did any work. What causes this? Any help would be greatly appreciated.