95 Sts Trans Overdrive

Has anyone had trouble with overdrive. 1st, 2nd, and 3rd seem to work fine. Overdrive feels like it goes back and forth to 3rd. Fluid is full, looks and smells OK. Happened suddenly, no warning like slipping or anything. No smoke, no noises. Any suggestions greatly appreciated.

Welcome to AF, to properly diagnose this a full scan should be done to check all the operating sensors as well as trans solenoids and slippage errors, this may be internal.