Frank,
We have similar experience before, however, we found it not related for
Ni/Au at all. As a poor PCB manufacture in Asia who have very little profit,
we have resharpen our larger size drill bit many times. This end up giving
us quite rough hole wall for NPTH which does not offend any customer spec.
However, this cause over plating of copper into the glass epoxy. Then,
sometime we need to avoid over etching for some products that have tighter
line width control, we have to run the board faster than usual this in term
cause copper remain in hole wall of NPTH. That's how we got Ni/Au in NPTH. 
Another possible cause for your problem could be catalyst remain in NPTH
hole wall, however, I'm not very familiar with chemical reaction. Try
increase the rinsing time before electroless nickel.

Regards,

Chris Wu

At 05:17 PM 1997/4/15 +0800, you wrote:
>
>
>Hello everybody,
>
>Now and than we are seeing that chemical Ni/Au is covering the NPTH
>wall. We and also the supplier of chemicals doesn't know what is causing
>this phenomenon.
>Is there somebody out there who does?
>Any help would be appreciated.
>
>Frank Petit
>ACB
>Belgium
>
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