External BB Crankset works loose
 

I am using a Shimano LX m-581 crankset with the external bb bearings on my commuter/touring rocket. The crank becomes loose after a while and develops side to side play that starts out small and gets larger. I first installed this set a year ago and after about 5000km the left bearing was worn out. Too soon in my opinion. Toward the end I was adjusting the slop in the crank weekly. So I change the bearings and pay closer attention to the tightness I apply to the plastic adjustment cap. Maybe I just got a bad bearing to begin with. Well about two weeks into the new bearings, I am getting the side to side play again. Shimano says to tighten the left crank in inch-pounds so I dont want to go too tight there. What gives? I am becoming a bit disillusioned with these cranks and am thinking about going back to the old style. Anyone out there have any similar issues or fixes?