Hi,
I thought that your G-Code file was changing. Your first description of the problem sounded like a hardware problem or an electrical signal getting into your system. In your second descriuption, you indicated that you were running at F50. Do you need to go that fast? Yes, offsets could be causing your second problem.
This is how I would approach the problem:
1. Make sure I have good electrical signal (no interference - motors on seperate line from computer).
2. Get a memory test program from the net and test memory multiple times.
3. Run Mach G Code program multiple times without anything connected to the parallel port looking to see if anything is changing in the GCode file or if the DROs and are showing the expected results.
4. Try running the code in a program such sa CNC Simulator (
http://www.cncsimulator.com/)
5. Go back to earlier version and see if problem goes away.
6. Remove any breakout boxes or Smooth Stepper and try cutting air.
7. Consider possible problem with servos or wiring - check and swap.
If I checked all those things , one of them should give me an indication of the source of the problem. The worst thing is when you have more than one problem. For example, it may be that you have to go back to an earlier version of Mach, do not use offsets and have poor connection to one of the servos.
Good luck,
Lee