DIAGNOSIS The friction washer or detent mechanism inside the speed control lever housing on the handle is worn or loose.
FIX Tighten or replace the friction components. Remove the screw or bolt that holds the speed control lever assembly to the handle. Inside, you'll find a spring and friction washers. If they are worn, replace them. If they're just loose, reassemble and tighten the pivot bolt until the lever has firm, distinct stops but can still be moved.
IF THAT DIDN'T WORK The cable that runs from the lever to the transmission might be binding or have too much slack, preventing full engagement of the speed setting. Check its routing and adjustment.
HEADS UP Don't overtighten the pivot bolt, or you'll lock the lever completely. Adjust it incrementally until it holds position under light thumb pressure.