I'll have to check to be sure, but I'm pretty sure my Tippet is fluoro and I use cinch knots as well (I guess it's technically the improvedcinch knot). All the breaks I can remember Sunday were away from the knots. I could tell by the length of leader left on. As for why else they may be breaking off, it may be for those other reasons you stated but it's pretty much impossible to know for sure.
The drag issue is something I really need to figure out as I've lost a good number of nice fish during those long runs. I guess I didn't say before, but it seems (and this may just be in my head) that the breaks come during the long run, but it's not a long run at one speed. The fish accelerates in the middle, and the drag (already spinning) cant keep up with the new pace.