I use a 2 flute upcut bit at 24000rpm at 6000mm/min and get good cuts but I would do an 18mm depth of cut in 6 passes with such a thin cutter a cutting depth off 6mm per passs will certainly cause the cutter to flex and I should think cause a bit of chatter as well. A 2mm cutter seems a bit on the small side if you are cutting right through, your feed speeds are also a bit low IMO but you cant push them too high at the depth of cut you are using.

Al