Originally Posted by
JohnDThompson
That's a swaged crank; those are more prone to flex than forged cranks. Depending on how much you want to spend, you might try a new or used forged crank to see if that helps.
I've noticed this flex on an old and swaged Sugino crank. Sheldon Brown has a pretty good description of what a swaged crank is
just scroll down to "swaging"