in order to avoid chatter with a long series that has that much stick out it is usually best to keep a high feed rate with a heavy chipload ,the tool has less chance to chatter if the cutting edge is well engaged into the material
what could be happening and is probably happening is the tool is pulling itself into the material rather than a deflect at which either way you'll get chatter , so to create that happy medium you could try dulling the cutting edge very slightly(very slightly) as well ,sometimes the cutter being too sharp will cause the tool to grab and cause chatter . i know there are those that would try to argue the point but i have had to do this with many tools on many different types of materials with great success , sometimes a sharp tool isn t necessarily a good tool for the job
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........