Every 20th frame is horked

[dgets]

There's something going on, probably with the following snippet, causing every 20th (and sometimes the following) frame to appear as if it's coming from within one of the bodies.

#declare ViewOrTransit = 1;	//1 for view, -1 for transit
#declare tmpFromMod = mod(frame_number, 20);
#if (tmpFromMod = 0)
  #declare ViewOrTransit = ViewOrTransit * -1;
#end

perhaps the mod statement isn't the way to be doing things here...