Weird Electrical Effect

[andywhy]

This used to happen on my Passat and it turned out that it was the aux tensioner being loose. After replacement it's not done it since. I do know some cars have electronic control of a lot of electrical loads so it sometimes pulses them when on the limit to stop the voltage going low and your battery draining. Though the 306 might be a bit old for that. My Passat certainly does it.