Thanks for all your suggestions. The line I have is fairly new, it's braided, and its rated at 25lbs. I really don't think that its my knot because the break happens before my swivel, I know this because I put a small weight above my swivel to keep my sliding weight from getting too close to the bait, and when I get this weird break I lose everything, so I know that it breaks above my swivel. I know that this must sound very strange to you, but I've never had anything like this happen to me before either, I am now thinking that its either a predatory fish (pike/muskie) or a snapping turtle. But I will try getting a bigger diameter line, and see if this solves the problem.
Paul