My 1997 Chevy 3500 2wd truck with a 350 ci engine and a T-400 automatic transmission sometimes doesn't shift.

Sometimes it runs and shifts fine and sometimes it wont get out of first gear. It will go but it stays in first gear and the RPM's get high. Reverse works fine. Its hit and miss every time I get in it I'm not sure if its gonna run good or act up. What could be the problem? Thanks!