Lighting and coolant warning problems!

Hi all!

We have a 2000 beetle and occassionaly the blue low coolant temp light comes on (apart from the usual warm up period at startup) even when the car has been running for some time and in mild outside temperatures.

The other "strange happening" is that when idling and cold when the headlights are on, the lights "Dim" for a second or so every 20 or so seconds.

I have measured the voltage when this is happening and there is a distinct drop in voltage as if the alternator drops charging every now and then.

Anyone know why these things are happening??? The dealers are no help at all.

Thanks, Mark.