Oh boy your machine is seriously out of whack my friend.!!
It's not unusual to see folks cutting Egg shapes which is easily fixed and down to wrong Step per settings in Mach3. But when the start and end point don't met again you have major problems.
First place to check is the machine frame it's self. Make sure Gantry is square to long axis. Check there's no binding or slop that will cause lost steps.
Next is to check that the 2 drives on the shared axis have exactly the same Micro step settings. Same in Mach3 both Steps Per and motor tuning settings should be exactly the same.
It looks to me like one of the shared motors is out of sync with the other causing the gantry to Rack. The fact this machine design is so weak and flexible isn't helping because it will be allowing a certain amount of Rack that would have stalled the motors on stiffer tighter machine.
Do you have home switches on the machine.? And have you set them so they align the gantry square to Long axis. If not do so ASAP.
For a test to see if the slaved motors are keeping sync mark each side or better still put a dial gauge on each side and check the both return to zero or there respective marks after moving back n forth several times.
This will almost certainly be down one of 2 things or mixture of both. Miss alignment/flex and/or loosing steps. Highly likely both.
Post your Mach3 settings for motor tuning. Also what type of linear setup the axis uses, ie Belt drive so pinion sizes etc.
Give some info on the electrical setup ie: drives setup and Voltage used.