getPrev and getNext stop working after 100+ entries Prev and Next stop working after 100 entries..  There are no template errors.  I can access the pages for the individual entries - but Next and Previous are blank.

Here is my set up...

{% set criteria = {section: 'posts'} %}
{% set prev = entry.getPrev(criteria) %}
{% set next = entry.getNext(criteria) %}

{% if prev %}
my html ...{{ prev.url }} ...my html
{% endif %}

The same holds true for next.

Once there was a template error referencing a null value on prev.url - and referencing an entry ID that no longer appears to exists.  However, I discovered that I had had prev under if next.  That is the only time I was able to produce a template error.
In your criteria, you need to set the limit to null as by default Craft only returns up to 100 entries.

See: Default Craft entry limit