Wouter van Ooijen wrote:

> I have no idea what a locum is meant to be?

Neither do I. :-)
But I guessed it wasn't anything positive... :-) :-)

> The fact that internal MCLR can cause problems is not
 > clear from the obvious sources,...
> 
> The explanation for this problem is complex,...

Exactly, that's why I thought that it was no use, at
this stage, to go into details about Vpp-before-Vdd
and other int-MCLR specifics. If you realy don't *have*
to have that extra I/O-pin, just leave it as ext-MCLR.
And follow the datasheet about how to connect it...

And I do not think that the OP currently need another
problem to deal with.

Regards,
Jan-Erik.

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