Spartan Router - Head drop issue

[Abstract]

Sorry for not describing it clear enough!
@EddyCurrent Basically with the last problem, when I returned the router to the work-piece coordinate (after hearing the horrible noise of the head dropping in to the board) I retested the Z height and it had dropped 10.95mm. I was using a V-groove with a 12mm head and 6mm shank at 3200 mm/min at 24000 rpm. I am cutting 2.6mm in to a 3mm thick material. To my knowledge, the spindle cut only in one direction.
Thanks for the reply

@phill05 I am sure that all components are metric, but thanks for the suggestion!

@JAZZCNC Sounds like a good test, so I'll do that now and reply with the results :D Cheers

[Abstract]

I ran the air cutting test a couple of times to see what was happening. The cutter did not seem to drop depth from the collet.
Once that was done, I thought about having a look to see if the material was ,in a way, pulling the cutter from the collet. So I cut 3 lines in some 19mm foamex with the V-groove tool used on friday. I started at cutting depth; 1.5mm, 2.5mm and finally 3.5mm. The head did not drop after all grooves were done.
With a bit more confidence, I risked putting some work through. I started with the basic route lines (which is where it messed up friday) and it seemed to complete it without a problem. The only thing I noticed was that the corner nearest the mechanical origin seems to cut lower than anywhere else on the board; why could this be?
Also, another question - how often should I clean out the collet and locking nut?

[EddyCurrent]

Sorry for not describing it clear enough!
@EddyCurrent Basically with the last problem, when I returned the router to the work-piece coordinate (after hearing the horrible noise of the head dropping in to the board) I retested the Z height and it had dropped 10.95mm. I was using a V-groove with a 12mm head and 6mm shank at 3200 mm/min at 24000 rpm. I am cutting 2.6mm in to a 3mm thick material. To my knowledge, the spindle cut only in one direction.
Thanks for the reply

Still not clear but you seem to be saying later on that it was the cutter dropping out of the collet.

The only thing I noticed was that the corner nearest the mechanical origin seems to cut lower than anywhere else on the board; why could this be?

Maybe the bed is not flat, it's usual to skim the bed using a fly cutter or large end mill to ensure it's flat. Make sure your 'sacrificial' piece of MDF or whatever is securely fastened down to the bed in enough locations that it remains in 100% area contact with the bed, give it a skim.

Also, another question - how often should I clean out the collet and locking nut?

I don't think this is something to worry about as it gets cleaned out each time you change a tool. The collet, collet holder, and collet nut need to be spotlessly clean in order to function correctly, any dirt could let the collet slip and may be where your problem lies.