I have no special ideas on this but Microchip has four or five apnotes 
on various oscillator topics.

If you go here:
http://search.microchip.com/searchapp/searchhome.aspx?q=oscillator
there will be several articles in the top 10 results that may help 
thinking about the problem.

Leslie, Shawn J wrote:
> I am using a PIC18LF4320I/ML. I am using Timer 1 and the oscillator as an RTC. I am driving the oscillator with a Epson FC-135 series 32.768KHz 12.5pF crystal good to 85C. I am hanging 27pF caps off the crystal. After 2 years of successful builds we are suddenly having this circuit fail at high temp. The oscillator fails at 60-70C and a few even do not work at room temperature. I am having 40% fallout at temp. If you lower the temperature to <40C it starts to work while power is applied. The problem is made more difficult since the units are potted. We cannot get at the components to troubleshoot. We are trying to test unpotted units over temperature. In the meantime I have tried to find a threshold of failure on a working unpotted unit by increasing and decreasing the 27pF caps - amazingly the circuit works at room temperature with 47pF caps on the high end to 6pF on the low end. With 6pF I can make it fail if I raise the temperature to 70C, but as it cools it begins to w
or!
>  k. I have wrote Epson for crystal ideas and sent them my layout - any ideas from you guys? What have you all used? Maybe someone has a successful crystal oscillator design I could use.
> 
> Cheers,
> Shawn
> 
> 
> 

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