Jerking motion and Clunking sound into gear occasionally

I have a 2004 Saturn LW with 6 cylinders. For the past few months, but only about one or twice per month, my car has been clunking jerkily when changing gears. There is no pattern, except that it happens at slower speeds, say mostly under 40 MPH both forward and in reverse, sometimes when I put the car into gear and sometimes when it changes automatically. Yes, you have guessed it, whenever I take it in, it acts just fine, and no one can get it to duplicate the bahavior.

Oh, the dashboard the the light indicating necessary service comes on whenever the gears clunk and the car jerks, but when it behaves, no indicator light comes on.

I have about 114,000 miles on the car. do I just need new transmission fluid, or is my transmission getting shot? Am I stripping gears? do I really need to wait until the mechanic hears it? Everything could be utterly destroyed by then, right? Advice from anyone?

My bet would be an intermittently failing pressure control solenoid in the transmission. When it fails the PCM commands max line pressure, which causes the harsh shifts and turns on the “service vehicle soon” light. Not to be confused with the “check engine” light. Generic code checkers will not help to diagnose this problem. Chances are there will be some P18?? codes stored which will help in diagnosing.