Beware of the EIS
My research on this forum and benzworld indicated that my electronic ignition system (EIS) was probably failing to recognize either of my keys. It was an occasional problem. The first thing I did was clean the sensors on the keys and ignition with a q-tip. It didn't work. The problem only got worse after I replaced the key batteries, which, as I've now learned, have nothing to do with the ignition system in a non-keyless-go car such as mine.
As the problem got even worse I began to wonder if I'd get stranded somewhere - the car was officially becoming unreliable at that point - so I took it to the MB dealer. He ran the codes and found no faults. Dealer said the car technically had nothing wrong with it so they couldn't order a new EIS - UGH! But fortunately the EIS system totally died right there at the shop. Codes, who need stinkin' codes!!
They replaced the EIS under the warranty and, as a nice bonus, I even got two brand new keys (the updated ones with metal trim - which match my GL450 keys. Now I have to label them.)
Long story short - if your ignition begins to stick when you turn the key, first see if the little red lenses in the key and the ignition are clean. If they are, get your EIS checked/replaced fast because it's only a short matter of time before it dies, about 1 week in my case. And once it totally dies absolutely nothing in the car will work. Apparently even towing it is very difficult.
Hope this helps someone down the line.
-qcar



