I bought my CNC router with Mach 3 configured with a USB connection instead of a Serial. Recently I started dropping Z height in Mid Operation. Checking bearings, slop in the screw, connections, and anything else I could think of is good. I'm running in a metric configuration, and when I calibrate it sets at 355 "steps per". Operation starts out good, but after several minutes Z height will drop 5 or 6 mm. The manufacture suggested changing the Steps Per to 160. At that configuration, things seem to be working better, but when I do a profile cut, the depth only reaches 55% of where it should go. Anybody think that having it with a USB setup might be my problem or is there any other advice?