If tightening the shift lever (increase the friction that it takes to move the lever by tightening the screw or loop that holds it on)does not work, it probably is a matter of lubricating or replacing the cables. Sometimes a shot of WD-40 in the end of the cable housings will do it but more often you must loosen the cable and pull out a short section to expose the part that is hidden within the housing. Then a tad of grease or a good wipe down with a solvent will free thinks up when retracted into the cable housing. If you have too much friction in the cable, it will not shift when the lever is moved (in this case loosened) and it will shift at some later time of its own choosing. Your rear derailleur is spring loaded to move toward the smallest sprocket and thats what yours is doing on its own. One of these two solutions should solve the problem. Good luck. Its a solvable problem. tom