Those early 2000’s Chevy trucks are notorious for having bad stepper motors in the gauge cluster, I don’t know if your ‘99 would be affected by this but I would start with this repair before replacing the fuel level sender. I’ve seen this problem and performed this repair successfully many times. I’m including a video of the process. It’s not shown in the video, but I always replace the LEDs while I’m in there. They’re typically included in your stepper motor kit. Good luck!

https://www.youtube.com/watch?v=mFApzUn5...