Voltage instability on Arduino MC can anyone help?

I am using an Arduino MC to animate 12 RGB LED's on the LPD8806 strip. The program works fine when drawing the power from the Laptop. However the only supply I will have when I use it is a rectified 6V ac dynamo input. The rectified supply works fine but the Arduino program gets fried because it does not like the gradual falling off of the voltage over time as the dynamo slows down. I then have to reconnect it to the laptop to upload the program again. I need to some how turn the voltage off when it drops below lets say 3V. Anyone have any sugesstions?

Have the rectified dynamo output charge a battery feeding the arduino. Loss of battery voltage should not scramble the program loaded into non-volatile memory on the Arduino. It will just shutdown if the voltage is too low. You need a minimum of 7v to use the onboard regulator. Using the 5v input requires tight regulation externally.

Since the processor should have a good undervoltage reset circuit you may have other problems. I'd look seriously at the voltage of the dynamo as it slows down, It could be putting out pulses that cause problems for the arduino reset circuits that are passing through your power supply. Better filtration might help - a 100 nf cap or even a boucherot cell (a 100 nf cap in series with a 2 to 5 ohm 1/4 or 1/8W resistor) might do it.

A battery should help, but there could still be fast transients. If you had a look at it with a scope you might see the problem. This may not be a useful suggestion - I realize most people don't have them in their back pockets.

I agree with Steve and Stephen that you need a battery, even just a small one.
Here is my suggested circuit, keeping it as simple as possible.
One diode prevents the dynamo discharging the battery at low revs, the other two drop approx. 1 volt to give a 5 V supply.
BTW, is your application a bicycle light ?