Bless you Peter for all of that very useful information. I changed out the fuel distributor, and driveability improved immediately. I now have a unique paperweight. Should be interesting to see if the gas mileage and cold starting also get better. I still have to check all of the vacuum lines you mentioned. I'm sure there's at least one small leak in one of them, given the age of the hoses.
Also, with regard to the cold start valve, I've been unable to determine exactly how it works.
1. At what temperature does it start to operate?
2. After the engine starts and goes into "warm-up" mode, does it stop operating until the next cold start?
3. Is it worth removing and cleaning it? It doesn't seem to be leaking.
1986 560SEC: 150,000 miles, runs great, but I've got to sell it (too many cars for one man)
1987 560SL: 122,000 miles, used to run poorly, now (thanks to forum), runs great!
1997 GMC Jimmy: Turned out to be a turkey.
1989 T-Bird Super Coupe: 150,000 miles, still runs great. Ford got it right.