hi mate
i had the same problem.
i was careless after getting a new crank arm and didnt tighten it up regular for the first few times. So the taper in the crank arm wore away. So i had to buy a new one. its the easiest thing to do...
after getting a new crank arm tighten the bolt up for the first couple of times then now and again. and always tighten up the pedal. trust me ! iam careless on these things, and ive spent too much money on cranks
thanks