Garry,
I've only used Pac Bay threads once and not metallic so I can't comment on the thread itself other than to say what I DID use loosened terribly when I applied CP.
It sounds to me like the CP you used didn't seal the thread properly before you put the finish on.
I have just started using Chromaseal and it's hands down the best I've used.
Shake it well before use.
It's very thick but dries clear without the snow on dark threads.
I've found it eliminates bubbles coming out from the layers of thread in 5 or 6 layer weaves.
ANY CP will darken your thread a shade, but the problem you describe suggests that epoxy has contacted the thread.
Also I wouldn't be sure that the metallic is actually NCP.
They are usually a metal film wrapped over a nylon carrier and given the way they unravel I doubt that there is any coating on them.