I'm thinking your breaking end mills because the chip load is pretty heavy at .0015 per tooth. Thats typical for for a 1/4 EM. The chip load per tooth I see suggested for 1/8" EM in aluminum is .0005 - .001"
It's also good to check the tool run out for such a small tool. The slotting parameters I listed has a chip load of .0006". If theres something like .002" of run out, that can break EMs.
Here's 2 videos I did on runout if interested.
[ame=http://www.youtube.com/watch?v=2rur6oFaYhY]TTS Runout - R8 Collet.wmv - YouTube[/ame]
Tormach PCNC1100, Mach 3 R3.043.037, MastercamX5 level 3.