This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: Check out the electrical connections, and battery (see above under
charging). Also check out functioning of the solenoid.
If the problems occur only with a hot engine, consider the VW
solenoid package that will cut out most other drain when starting,
or the heat shield package (also from VW) for the starter.
 
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