How can I pulse one of several wires in Minecraft randomly?

Have a pulse going around in a loop. Along one side, take the first repeater and set it to the longest time delay (for heightened probability), followed by 1 redstone, then the rest of the loop.

Should look like this:

Key:

I = Long setting W = Redstone wire

Current is anticlockwise

wwwww
w   w
w   w
w   w
w   WWWWW
w   I
wwwww

Here the line of W's is line A.

Repeat with B

Next, repeat with C and D, but with shorter delays. (Probability)

Leave E without an extra delay.

Next, make an AND gate on each line. One input will be the button, the other will be the line. The output of those are the lines A, B, C, D and E.

Please comment if you don't understand and I will amend as appropriate.