Bo:

When you say you have a known good glow plug; do you mean
that the same plug has been used since new ?
After break-in, the plug coil gets a baked on residue on the coil.
It will still glow as if good, but that white color means it's time for
a new A3 plug. What you describe as symtoms are classic plug
changing time clues. When you advance the throttle and dump in
fuel that the plug is not hot enough to burn. Did you notice that
the engine slows or stops when you remove the ni-starter ?

Randy