Rough Idle 94 E320 need diagnosis

[TomO]

My wife's 94 E320 wagon (120 k miles) suddenly developed a rough idle yesterday morning, in town, 5 miles after it was started. She drove it 25 more miles, mostly highway, no change and no check engine light. That afternoon I drove it back home. It started fine but idled rough, and the check engine light came on immediately. It seemed to shift very hard the first few miles but maybe got better after warming up. I drove it home (30 miles). On the highway it ran OK, but maybe a bit rough intermittently, and seemed to lack power.

I've searched archives--tonight I'll try to read the engine codes and will run the car with hood open in the dark to look for arcing. Any other things I should try to do?

We bought the car last year from a dealer in a neighboring state at 90 k miles, and had major service performed then.

Thanks