Jack:
You checked to see if moisture droplets were around xtal, cps, leads, and so on?
Before using heatgun can you get to the caps on the xtal? If so, then grab one and warm it with your finger heat and see if the xtal kicks in. I have had bad xtal bypass cause the 168 to shut down in cold. If you could get a scope loop near the xtal to see if it is shut down or an rf probe. I does sound like the 168 is not getting clock by all you have said.
It would be nice to see the culprit! If the data stream stopped the darn thing has to be not getting clock - unless
you have a bad 168!
I will be lurking....