>Also, how can I test the xtals?

The easiest way to test them is to have a spectrum analyzer with tracking
generator. One manufacturer (Marconi) had an app note with a circuit described
as follows (my ascii art is hopeless)

Put a 50 ohm load on the tracking generator, and a 50 ohm load resistor at the
end of a coax lead to the analyzer input. connect the crystal between the active
ends of the 50 ohm loads. run the analyzer and tracking generator through the
frequency of the crystal, with a sweep of +/-5khz (probably for 18Mhz), and you
should see a signal on the analyzer which looks a bit like the graphs for phase
response. The lowest amplitude point is the parallel resonant frequency of the
crystal, and the highest response peak is the series resonant frequency. The
depth of the dip is a function of the ESR of the crystal, (remember at parallel
resonance it will be seen as a parallel high value resistance) and I would bet
that the crystals that do not work will no have a deep dip compared to ones that
do work. This ESR figure will also show in the height of the series resonant
peak, in that a high ESR will limit the height of the peak.

If you have the time, you could do a similar arrangement with a signal generator
and an RF voltmeter or oscilloscope, manually tuning the generator to look for
the min and max response points. You will see several of each, and need to pick
the highest and lowest points to get the relevant ones. If you really want to
get fancy, it would be possible to calculate the ESR of the crystal by knowing
the voltage division ratio into the 50 ohms on the output side of the network.

From this I think you will see that the problem you have is the crystal
manufacturer is not controlling the ESR of the crystals (probably because there
is no specification) and Microchip are not controlling the spread of gain and
phase characteristics of the oscillator, so sometimes you end up with a
combination where the oscillator gain is insufficient to overcome the ESR loss.
Your suggestion of adding a series resistor will only make this worse, and
should only be used with crystals which have a low ESR or oscillator with very
high gain, to stop the crystal being overdriven.