Just a few ideas I thought I'd throw out:

* I recently found the coil wire/rotor contact inside my distributor cap badly chipped and possibly causing arcing/spark scatter inside the cap. Rotor top had burn marks. But I haven't run it yet to confirm.
* Reluctor gap change?
* Old plug wires?
* Choke pull-off/hose leaking?
* From what I've read about the sensitive Thermoquads, expect the unexpected. (Still learning.) Do you have a good, spare carb to try?
* Any unusual wear indicators on oil pan magnet or in trans pan?

I guess if it's been gradual over a few years, it's not any weather anomalies.