The computer I use to run Mach3 needs a complete hard drive reformat. Can anyone tell me if there's an easy way to save all my hard-earned Mach3 settings and configurations so that when I reinstall the software I can import the settings and restart where I left off? I was thinking as a precaution I should take screenshots of all my config pages but isn't that what the xml file is for?

As you make changes in your software configurations, does the xml file automatically update itself? 

Any advice would be gratefully received.

Yes, just copy the xml file for your profile and it should go ok. Make sure it is the one for the profile you have set up as you may have made up a profile when you started, If you didnt then it will be Mach3Mill.xml.
Hood

Many thanks Hood - I'll save the xml and use that to set up when I'm done.

OK, I reformatted and re-installed Mach3. Everything seems to work as far as the xml configuration is concerned. The only difference now is that the pulse rate according to the diagnostics screen is unstable. It varies from about 4K to 28K. When it's stable the CNC runs fine but as soon as the pulse rate starts bouncing around the motors can't run properly. Any solutions for this?

Sounds like you have a process or program interfering with Mach. Do you have any programs on this time that you didnt before? Quick time is one to look out for I think as it is known to interfere with Mach. It might be worth doing the optimisation steps, you will find them on the downloads page.
Hood