If you're trying to make 3D cuts in rotating tubes, it sounds like you're really pushing the limits of SheetCAM, which, as the name suggests, is intended for 2D work on sheets of material. I'd check the G-code for any commands starting with "F" - those control the speed the machine's supposed to cut at. If you've specified 80 ipm and it's going at 200, it's either assuming it's out of the cut and making a rapid move (also look for "G00"), or it (or your controller) is going haywire. From your description of the movements the Z axis is making, the latter shouldn't be ruled out.