Walker shear pin question

[bart may]

I recently bought the walker with a 56" side discharge. I'm finding that after 2 hours or so the shear pin on the pto (the one just before it enters the blade drive line) will break off when I engage the blades. I've tried lowering the rpms before engaging but it still only last a little time. Have any of you had this problem? Can I get a little tougher pin? Is it because of the size of the deck and number of blades? Thanks for help.