P0122 OBD2 Code in 1999 Chevrolet Alero: Hard Shifting, Fuel Gauge Inaccuracy, Radio Reboots & Ignition Switch Recall

I own a 1999 Chevrolet Alero with 164,000 miles. It has been running reliably until approximately six months ago when I began experiencing hard shifting during gear changes. A transmission fluid change previously resolved the issue for about six months, but it returned after a 30-minute freeway drive—this time occurring as I entered the freeway ramp (a first occurrence since the last fluid change). The car had been cruising on backroads at 45–55 mph before transitioning to highway speeds. The next day, the vehicle ran normally. However, five days later, while driving on the freeway in stop-and-go traffic at around 65 mph for about 30 minutes, hard shifting resumed. During this instance, I noticed a whining noise from the engine and transmission, which I believe indicates restricted fluid flow or excessive pressure buildup. Shortly after, the vehicle began running roughly, and the Check Engine Light illuminated with code P012-2. Additional symptoms include: - Fuel gauge malfunctions: Over the past two months, the gauge has fluctuated—dropping from 3/4 to empty (then returning when restarted), then reading significantly higher than actual fuel levels (e.g., showing 90% full when only about 40% remained). These issues worsened during episodes of hard shifting. - Radio behavior: In the past month, the radio has started performing a 'reboot'—attempting to eject CDs or tapes, displaying an E11 error briefly, and then resuming playback from the last channel with 'Manual' displayed. Preset channels remain functional. I also have an active ignition switch recall from GM, which I plan to address soon as part of a scheduled repair. Given that this recall involves electrical shorting in the ignition harness, I am wondering if it could be contributing to the P0122 code or other symptoms. Note: The brake light and antilock warning lights remain on due to a saturated brake reservoir float—this issue was resolved by manually adjusting the float. While brakes function properly, this has been an ongoing condition for over a year. I believe it is not a safety concern but may warrant replacement if needed. I am considering whether the P0122 code and related symptoms are directly linked to the ignition switch recall or if they point to deeper transmission or sensor issues. As the vehicle is 16 years old, I'm cautious about costly repairs and prefer to explore potential root causes before investing further.