I have the ShopBot Desktop with Z height kit added with a spindle (not router).

For some time now I've had issues when performing a profile cutout. It's somewhat erratic, but on occasions the bit will slip down about 0.5". The bit is a 0.25" sipral upcut bit. I have no problems with other bits (0.125" spiral upcut, ballnose etc), only the 0.25" spiral upcut. During the execution of the profile cut out, the bit drops and starts making deep cuts.

What I've tried:
I've measured the diameter of the bit, and it is consistent with a new, unused bit of the same make and model. I've swapped the collet for a new, unused one. I've made sure everything is blown clean and clear of any dust. I don't insert the bit so far that any part of the flute is inside the collet, nor does the top of the bit pass beyond the top of the collet. Previously, I've tightened the collet nut the same as I would for other bits. I have also tightened it with greater force, trying to ensure a better hold, but still the bit has slipped.

Other than swapping the bit for the new one, I've no other solution.

Has anyone else experienced this? Suggestions?

My good friend Scott would probably say it's because everything is upside down in Australia. He may be right

Thanks

Graeme