Does a different key operate the car normally? If so, that would point to a failed electronic section of the key in question.

How bright are your headlights? Do they dim significantly when you try to start the engine? A crap battery or poor connection can cause all sorts of weird problems, especially with the high current draw of the starter motor.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]