I have found that when a key starts to wear out, you will get intermittent false alarms when opening the front doors. On my 86 300sdl, one of my master keys will always set off the alarm. I have reverted to using my other master key. I guess after 24 years things start to wear out.
On my 85 380se, I had to have my ignition lock cylinder replaced and I went ahead and replaced the door and trunk locks also so that I could use only one key. The new key from Mercedes does not look as sturdy as the original key. I bet it will not last as long.

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John
1986 300sdl
1985 380se