Spartan Router - Head drop issue

[Abstract]

Hello everyone,

New member here with an issue concerning my Spartan PJCM 1325-3. Basically, my boss brought this router about 3-4 months ago to help with our sign production. With one day training, we were able to work the router with no issue.
Unfortunately, a week down the line we started getting a problem where the Z axis would just drop, ploughing through the sacrificial board and sometimes in to the polymer base! The odd thing is that it will go from cutting 0.3mm at the start of the vector to cutting down to 15mm.
An example of the Z axis head drop. I was cutting an Acrylic letter B out.

Once this happened we phoned up Radecal Machines, whom we brought it off, and told us to reload the parameters. Once I had done that, I set a test run to cut out a Dibond star - this resulted in the head dropping again and snapping the tool. So I re-installed the parameters a second time and it seemed to work fine.

This started to become a trend where every couple of weeks it would randomly drop the Z axis. After about the 5th time, Radecal sent over a new hand controller, new 6mm collet and locking nut. Once these were installed the machine started working perfectly with no errors.. until today..

After cutting around 90% of a net tray sign, made of 3mm Dibond, the head decided to drop on the final vector whist still displaying '3.3mm' on work piece coordinates . Once I got the machine to return back to it origin and switched it off, I noticed the drill piece had dropped - 10.95mm to be exact!
Now, what I do not understand is how; firstly it got rough 90% of the way through without dropping and secondly I had tightened it up to how I always do.
I have had little experience with CNC machines, but am a quick learner. So, my major question is what is causing this random Z axis drop! Hopefully the pictures should possibly give you an idea on what I'm talking about.

Thanks in advance,
Adam