I have experienced a similar issue. It only occurs when laser engraving a large file (+30MB ?), and only when the file is finished on the final X0/Y0 moves. I use Windows 7 64 bit/4GB and a UC100 USB adapter. My suspicion is perhaps Mach3 is resetting the buffer to the beginning of the file by reading from the hard drive. This is only a guess as it does not happen often enough to confirm, and never happens during engraving. Driver test is good and solid.
Any commonalities?
John Champlain