There are 2 things that can cause this.
1. There is a faulty diode in the alternator.
2. (more than likely the cause) voltage difference between the alternator and the warning light wiring which is a common fault with older cars.
Have someone check your charging voltage, if its down there will be a faulty diode. If its not then you can either go over all the connections in the wiring loom to rectify the problem, or just put up with it!
Woz