About 5% of my production run is having
problems with start up of the oscillator.  I am
using a 4 MHz parallel crystal Digikey part number
X006-ND, with a pair of 22pf NPO ceramic chips
on either side to ground.  Local bypass is good.  At first,
because the board is not accessible to customer, I had
been leaving the flux on to save time and environment.
 For normal resin flux, this didn't seem to matter, but I recently
switched to organic water soluble flux, and I thought
I would pass along my tentative observation that this stuff
has got to be thoroughly washed off.

But even with a clean board, the start-up problem
persists.  Has anyone done any comparison between
quartz crystals and ceramic 4 MHz resonators regarding
reliability of start up?