I'm posting this in the hopes it may help others with similar intermittent starting problems. My car is a '92 1.8L Digifant Golf, but I expect the problem is common to other models.
Over the past year the car ran perfectly, but became increasingly difficult to start. The engine would always crank but usually not even sputter. Turning the key to the "ignition on" position repeatedly before engaging the starter provided a temporary cure, but recently even that trick wouldn't work reliably. After much searching for the problem I connected a DVM between coil terminal 15 and ground, and found that the voltage on the coil was dropping almost to zero when cranking. Disconnecting the starter motor didn't change this result, showing that the problem was in the ignition switch, not in poor connections to the battery dropping voltage when the starter drew full current. At present I'm placing a jumper wire between battery positive and coil terminal 15 for start-up, and now the engine always fires up almost as soon as it starts to turn. A new ignition switch is going in this weekend.
Garry