OK, How about "Buy a Wilson"?
Does that finger look different (deformed, dirty, etc.) from the others?
If you do not use the neck pilot, how much wobble is there at the cutter end? It could be a bent collet or misaligned some how, such that when you put in the neck pilot, it forces the case out of the collet after you start turning it.
Forster recommends that you put the pilot in the neck before tightening the collet on the case. Does that help with yours?
Hope this helps, but if you want to replace it with something else, I highly recommend the L.E. Wilson trimmer. No neck pilots needed. Case holders work for different cartridges that share the same basic case body (i.e. 243/308 is one holder, 270/30-06 is another, etc.) Sinclair sells some really nice accessories for it, but they are not necessary, and without them, the Wilson is actually a pretty inexpensive trimmer.
Andy