Gigi,
Thank you so much for your answer.
My mat is most certainly a little warped, but I rotate it often and can say with some certainty that at least some of those cuts are falling in flat sections of the mat. But now that you've alerted me, I'm going to pay attention to the location of the cuts and extent of the double line on my next go. Brilliant suggestion, thanks!
My new order of 60 plus blades just arrived (the ones I have in stock are dull) so I will try with that.
As to the location of the problem cut, it seems to be on opposite sides of the rectangle so if on one horizontal it falls on the top left the on the second side, it would be bottom right.
My velocity is about 400 and the force 200. Blade extension is approximately the same as material thickness and blade height is set with the felt gauge that shipped with the machine.
One more thing I noticed, the problem does not occur on curves or complex cuts, just on the long horizontal straights.
Trust me to get the weird problems
Thanks again Gigi, I hope the added info will help you think along so we can get to the bottom of this problem.