I've been running one of these mills for over a year. I've experienced the "stabbing" tool sensor also. It really seems to be related to a "weak signal" as stated above. Usually the stabbing happens on the small, .6mm bur. Which in my opinion, makes sense because it has the least amount of contact area with the measuring plate.
Ive done a few things that have improved the situation:
1. Fine zirconia powder will work its way UNDER the tool plate, which I think can interfere with its grounding. When I install a new plate, I put a bead of Vaseline around the edge of the plate to seal it from dust. Also, there are two small holes on the bottom/side of the a-axis plate that intersect the two screws that ground the tool plate. Zirconia dust can penetrate that area as well. I filled the holes with hot glue to seal them.
2. Make sure that both the collet, and the spindle are CLEAN. I have a small inspection mirror that I use look into the area where the collet threads. Zirconia can get built up in that area pretty quickly. I clean it regularly with a pipe cleaner.
3. Next time this happens (when, not if) I plan on adding a secondary ground strap to the tool plate.
Also, if the mill is in a high dust area: There are no air filters on the cooling fans!! The inside of the mill can get caked with dust. After the 6 month mark I started having weird issues with the mill. We took the case off, and meticulously cleaned it, and double checked all of the chassis grounds. After the cleaning the mill performed excellently. I don't know what the correlation is, but cleaning it made it work better.
Concerning burs: I call BS on mfg's blaming problems on the bur. If the bur can conduct electricity, it will work. Period. I've used Oem burs, cheapo burs, and currently use sierra tools full time. I've had the tool plate issue with ALL OF THEM. I've run the sierras into the ground.. like 70 hours of run time per..
These are my thoughts...
-Greg