[Spinnetti]

I'm struggling to not get chatter on a long piece I'm trying to machine. Its made from 1.25 x .5 x 9" 6061 rectangle, and is machined on all sides. I'm using two vises and holding it fully on both ends, and cutting a taper over the full length where its deepest at the ends and shallow in the middle. I'm getting hit or miss results with a lot of chatter. I've tried deep cuts, shallow ones, fast and slow (using G-wizard, plus experimenting).... Any ideas on how better to fixture it?