I have tried something like this before with flapping objects and I did not want them to flap exactly at the same time. What I did was I just moved one set of key frames over a few frames so that they did not land on the same spots at the exact time. Now they would still MOVE at the same rate/speed but one would either be more ahead or behind the other.
If you want them to randomly change rate/speed, so that sometimes the left is faster than the right or the right faster than left at times that might be a more tricky to do IMO.
I assume since your using a circle path to have the Nulls rotate around there should be a few points in the path. Maybe turning a few keyframes into easy ease? ease in or maybe out so that the rate is slower/faster on some parts of the circle vs others?
Hope this helps.
Visual Effects & Motion Graphics Artist