Layer shifting on product printing after raft support structure First post here, so please forgive me for any silly mistakes.

Recently the y-axis (I think, forward-backward) has started skipping forward when printing the first layer of my print. It prints the raft with no issues at all, sticks the bed nice and flat and the raft is perfect every time. 

Then it starts to print the actual item, but every time it starts to the print the back right corner the Y-Axis jumps or skips on the gears and makes a loud grinding noise. The amount it jumps varies on the print, but always jumps towards the front of the print. It doesn't matter on the size of the print or the position, it always jumps forward never back.

The printer is a Makerbot Replikator clone, from eBay. The X &amp; Y moves the extruder. I have tried upgrading and downgrading the software (which uses Makerbot software) I am unsure if I can update the firmware.

Any help much appreciated.

UPDATE:
Image has been attached of the failed part, excuse the colour, I use it for my test prints. This is orientation it printed on the bed. First 3 blocks printed fine, 4th one jumped  It printed in the following order 1. top-left, 2. bottom left, 3. bottom right, 4. top right.
Layer shifting is a result of the use of open-loop control systems. This means that the printer just instructs the head to go to certain positions without checking that it actually did arrive at that position. If something happens along the path, like hitting some part of the print or the printer, the motors could loose steps or the belts may skip teeth without the printer knowing it, so it continues further without correcting this.

There are a couple of causes for the skipping to occur. Usually it is a mechanical issue, but it can also be related to an electrical problem or a print settings problem (if e.g. your print speed it too high, the steppers could miss steps).

Looking closely at the image, you will see that the infill of the first layer does not touch the outer lines. This could be a hint that your belts are not tight enough (mechanical issue). Too loose belts cause the stepper driving pulley to skip. Also check the pulley whether it is securely fastened so that the pulley is not slipping when subjected to an increased torque. The nozzle most probably hits the raft (ABS? as it curls up and detaches, you see some irregularities on the left of the bottom right product in your image). Do note that too tight belts are also not wanted as they stress the stepper by an increased torque load. If the issue is electronically related, you could thick of increasing the torque by increasing the current through the stepper (and driver). Too much current will overheat the stepper driver though, so make sure that these do not exceed their rated maximum and properly cool the stepper drivers.

To solve your issues it is recommended to first try printing at lower speeds (this can be skipped if you already have low print speeds), then check the mechanical system of the printer, and finally, if this does not fix the problem, you could look into the electronics.