Really Random 1/4" x .03 Corner Rad. 4FL AlTiN coated End Mill Breaking!

For over a year now we've had this end mill randomly break. This end mill runs 200+ parts cutting four .30 wide slots, .30 deep per part using a trochoidal tool path. It's currently running at 8500rpm at 40ipm in ductile iron. This will run great for about 6-8 weeks at a time then suddenly start breaking after only 3-20 parts. The operators have reported it breaking multiple times in a shift.

The end mill is currently held in an ER style tool holder on two different machines (Mazak Integrex and Mori Seiki NL2500). I've had the operators replace the collet with every end mill to insure it's not damage from the collet that's causing the breakage. Also, ironically, they break on off shifts.

We've tried different feeds and speeds. Then we've tried 2, 3 and 4 flute end mills with similar results. We've also tried different brands. It was suggested that it may be the different material lots that may have a slightly different makeup but this has started in the middle of a mill heat lot so that was unlikely. There was the idea that the coolant is warmer in the afternoons/evenings and the air temperature hotter so something about that could be causing the breakage. But then that doesn't make any sense on why it could run for weeks at a time with no issues.

Has anyone ran into anything like this? Any suggestions?

my two cents is it is a difference in the metal for a few parts , but I have also seen bad batches of endmills too . Also yes machines run differently after running for 13-16 hrs , controllers heat up and act up !!!
I have seen my share of things that only happen after first shift .