Hi, Cuemaker
Post the G code file,
If it was a lic. issue you shouldn't see any scrambled lines, Just nothing displayed after 500 lines of code, I think.
It may be your initial setting's in Mach3 or a Mode change not being set to proper state by your CAM post such as I J's ABS/INC G90.1/G91.1 or ABS/INC G90/G91 after line 500.
Chip