I don't have GWizard in front of me, but a couple of initial thoughts. Use a 2-flute EM to help clear the chips. It's very easy for a 4-flute to get clogged when slotting like that. If the end mill is HSS, you really need to look at tool deflection. GWizard is great at that. If you're bending your bit, it's going to break sooner or later.
If the part is aluminum, make sure you're running coolant or fog buster. Aluminum will get gummy and stick the the cutting edge, making it dull. This breaks bits real quick.

If you haven't already, I highly recommend reading the feeds and speeds article at cnccookbook.com
CNC Feeds and Speeds Cookbook and Tutorial