Steve,
From the online forum searches that I did a few years back it seems that the way the blade and handle interface comes together it allows the knife blade to wander a bit.
Yes I’ve tried new blades, old blades, rotating the cutting base, everything it seems, but I still get inconsistent results. I’ve cut thin 0.010" to 1/4" thick styrene and it seems to exhibit the same problem. As I said before it is not noticeable in the thinner pieces, but manifests itself much greater in the thicker pieces.
Here’s a couple of threads…
http://s362974870.onlinehome.us/forums/air/index.php?showtopic=243886
http://www.zealot.com/forum/showthread.php?t=98930
http://www.mylargescale.com/Community/Forums/tabid/56/aff/24/aft/125005/afv/topic/Default.aspx
http://www.britmodeller.com/forums/index.php?/topic/234911277-nwsl-chopper-blade-deflection/
http://cs.trains.com/mrr/f/88/t/152000.aspx
Craig