Jack's SL500

My 98 SL500 ran just fine all summer and fall. It has been out in the cold (mostly below freezing) for the past two weeks. I went out to drive it yesterday and it will hardly idle (6-700 rpm) and is real rough and shaking. Does not want to rev at all until it gets above about 2000 rpm when it runs somewhat better but is still rougher than normal. At idle it feels like half the engine is not running. Tank was 7/8 full of month old gas with stabilizer in. Ran fine then sat out for 2 weeks and now a mess. Any ideas where to start?

Bigdot

gas, plugs, wires, cap.