I've had a similar problem on my w140. I discovered the lower latch was recessed to low for the catch to successfully engage. And after slamming the trunk lid to help the latch meet the stiker it would eventually slip agian. After moving the latch up a few centimeters, things were back to normal again.

I've attached the MB procedure for adjusting the latch.

The last photo Trunk3.gif #7 was the fix for me.

Attached Thumbnails

     

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Russell McMahon
Tulsa, OK
2001 CLK 320
1992 400se
1993 400e
2000 320ML