To clean the epoxy out of the threads, run a tap through the insert. It will clean the threads right up.

As far as your inserts not staying put, one of the biggest culprits is not properly cleaning the inside of the shaft before putting the insert in. When you cut the arrows, the carbon dust will coat the inside surface of the arrow and can lead to adhesion problems.

With respect to the whole epoxy thing, I don't use it. I have been using low temp hot melt and have yet to have an insert come out. The advantage is that you can gently heat the arrow and get the insert out if you need to.