Hi John, I suspect it might be your computer, however if your in the UK the weather might be the problem. My machine is in the workshop (shed) down the garden and has been working without a hitch most of the last 3 years however during the cold spell things didn't work all the time, due to condensation from my heater or being below zero. Now the hot weather is here my machine has started missing steps in the X & Y axis, you can here the motor stop and then jump even when not cutting. The colour change to the G code screen is the clue, your computer might be overheating and causing the graphics card to default. Try it in the morning before it gets too hot. I tried my machine at 6 this morning and all worked fine, its now 3 in the afternoon and 30 degrees, (its 36 in the workshop/shed) the machine is playing up again, the temperature must be affecting the 5 volt controls. one other thing to try is the wire going into the BOB for the z axis down, has it come loose? JUST A THOUGHT.
Hope this helps