Both of you are probably right, although I have a feeling that its probably not that difficult a problem, its just finding it. Unfortunately, the diagnostics on this system are pretty rudimentary, and I'm told that each component in the system has to be tested separately. If this is the case, its likely to cost $$$ at the dealer.
Oh well, the cost of owning this car is far outweighted by the pleasure it brings me.
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.