I had a 2000 Jetta GLS (NA 2.0L) a few years back. Around 95,000 miles it began to have an overheating problem which was, after replacing the thermostat and the removal of about $650 from my bank account, determined to be caused by a broken plastic impeller in my water pump. It was replaced by my mechanic with a metal one. I'm pretty sure I remember him saying that the shaft would spin freely once the temperature reached a certain level and, since the impeller wouldn't be spinning with the shaft, no coolant was circulated.
I don't know if they changed the specs on the pumps with plastic impellers since then, but I've been extremely wary of them since.:eek