I have been using my "Hobby CNC" based router for several years during which it has cut hundreds of balsa wood components for model airctaft kits and models using Desk CNC software.
The machine has recently developed a rough-sounding and erratic movement at the first reverse direction motion on the Y axis. This varies in intensity and creates the effect of progressively forshortening the toolpath to the extent that the return to zero/zero at the end of a 20 minute cutting file, the cutter can be up to 30 mm from the initial Y zero point.
This is a very simplified description of the condition which has been posted to determine if someone may recognise what is happening from personal experience and provide assistance in diagnosis and correction.
I have attached a picture if the machine for reference.
I reallize this may require much more detail if the problem is difficult to diagnose, but a simple starting point is probably the best means of introducting my plight to this vast forum for examination.