560 sel idle problem too high,rough
It sounds like a lot of air leaks. It most likely is from leaking seals around the fuel injectors. Plan on removing all the fuel injectors (while they are out, have a good shop clean and test them all for you--cleaning them with a good solvent flush can work wonders for bad spray patterns). Then replace all the O-ring seals on the injector holders, the seals on the injsctors, and any of the rubber air piping that has hardened (probably all of it) . A lot of individually small leaks creep in and then cold weather opens up some more and VOILA! a high rough idle results.
Parts are inexpensive, and time is not more than a couple of hours.