Intermittent no spark for start, 1998 Merc Sable Wagon (cold only)

This 1998 beauty of a wagon was my late father’s car; he put 30K miles on it during the 10 years he drove it in California. It mostly sat in his garage. I now have it in Mazama, WA. Last winter I dropped it off one night at my local shop for some service the next day. The morning temp was about 10F or lower. Mechanic called and said he could not start it–said car would crank but there was no spark. This had never happened before–car always started on first crank. They found error codes in the computer saying the security program for the keys had dumped, so they reprogrammed the keys. It started fine. I picked it up, drove to local gas station (temp at about 0F), it would not start again, and the guys towed it back to their shop. We ended up replacing the PATS(?) module on the steering column, hoping for a fix. Three weeks later, after some days of cold and rain, car would not start on my driveway. After 3 straight days of crank and no start, I punted and had it towed 2.5 hours to Ford dealer in Wenatchee (thanks, State Farm)–where it started fine for the 2 months they kept it and did nothing to it. They said there were NO error codes in the computer, and that I could replace the main computer, and/or the main PATS (security?) module, each of which would cost $1000 or so, and both of which might not solve the problem. They claim to have called a Ford main tech line and got no suggestions. I have the car at home now and it runs fine, but I am afraid it will strand me next winter at one of our remote XC ski trailheads. Any ideas??? Coil and plug wires all test fine…