[sanddrag]

Other folks above have beat me to offering the likely root cause of your issue, but I'll also throw this tool out there for Feeds and Speeds http://zero-divide.net/?page=fswizard

We use it exclusively with excellent results for most types of operations.

Also, I will second the necessity to use an "Adaptive" (aka: trochoidal, constant-engagement, "high speed") toolpath whenever possible when milling. It's highly preferred, for a variety of reasons.