I had the same symptoms up until 2 days ago. In addition to erratic lock behavior, I could also hear a hissing of air in the trunk.
Turns out that a vacuum line needed a bit of rubber tubing. A 1 inch section had rubbed trough, or was possible eaten through by acid when my battery exploded a while back.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D