My 85 300D does something like this.

When I put the car into park and if I let off of the brake the car will slide and then I can feel the parking paw grab the car to a halt.

I've been using my e-brake when I park my car instead of just relying on the transmission. It's a habit I got myself into and I don't remember how or why I did.

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-Typos courtesy of my mobile phone.