What is the most common problem with automatic transmission with LS1 after 20 y

Hello, honestly - your vehicle is 21 years old with an untold amount of mileage on it. There is no "common problem" list for a vehicle of this age -- just about anything can be a problem. You could have internal transmission wear, clutch issues, band issues, valve body issues, shift solenoid issues, torque converter issues -- really, i'd suggest having your vehicle checked by a local shop or transmission specialist near you. Have you checked the fluid level and condition? www.repairpal.com/repair-shops