Hi, Andrew.
There is a whole Synchronar thread devoted to the xtal oscillator and the capacitors... Each type (cut) of Xtal has a designed load (Capacitance) associated with it.. In my case a standard load of 12.5pf (Note - I use the small cylindrical (Watch) xtal's to replace the standard ones fitted.)
http://www.dwf.nu/viewtopic.php?t=2944
Yes, i have two modules like you describe, but in my case i am 90% sure it's a dry joint that i have missed - when i have the time i will go over every joint. The other assumption i have is that in these modules the oscillator is on it's limits of working and just simply needs a change of series resistor, load capacitor or indeed xtal to bring it back within the operational window. But that's a lot of trial and error, and reserved for when i have a bit more time..
And as for my temperature sensitive one, i brought it in from the cold and it has not stopped yet.. So i guess that will require a bit of experimenting with values to make it reliable, or try another crystal..
It keeps me of the streets..