Sometimes when you put in a fresh insert you get chatter because the new cutting edge is so sharp. Was the previous insert that did not chatter used before you started this job?
A possible fix is to stone the sharp edge of the insert to put the most minuscule chamfer on; a couple of swipes with an oilstone might do it. The idea is to take of the extremely sharp edge without dulling the tool and destroying its cutting ability.
An open mind is a virtue...so long as all the common sense has not leaked out.