After thinking a bit about the Stability Issues with the robotic leg I decided to do some more experiments and try to reproduce it at various angles and orientations of the leg mechanism. I Rotated and Extended the leg to get it into a position that was causing a great deal of instability then held it in my hand. I tried rotating it upside down while poking it with my finger to get it to resonate and shake. It seemed to do that in all orientations and positions... so I get the feeling that it isn't just the balance issue although that is part of it. While moving it around I noticed that applying some pressure on one of the servos, essentially fighting the motor caused some jitter in the other one. This had to be a power supply issue so I put my oscilloscope on the power then signal wires and could see the jitter in not only the servo position command signal but in the power supply as well.... Looks like the little Futaba servos pull a lot of current and are messing with the control circuit I built. To isolate the problem I separated the power supply wires for the motors and the control circuit and then powered the control circuit from my home made triple output bench top power supply.
That seems to have fixed the servo jitter problem for the most part! There is still some instability but that appears to be related to the servos and inherent in their design (still learning about these) I should have run a dual supply from the beginning but I'm thinking long term as in a mobile Robot device that would only have one power supply for everything. Although I am reconsidering that now at this point!
In summary I believe that what was going on is a combination of several things: 1) The servos are a bit jittery due to their design and low cost 2) The natural resonating frequency of the leg is near the 'jitter frequency' 3) The single power supply and the common wiring was injecting noise into the control circuit 4) The noise caused the servos to jitter and oscillate more 5) Because of number 1 and 2 above the entire system was feeding back on itself 5) Certain orientations and balance exacerbated the problem.
I'm going to test for that and be sure that the leg will function in all orientations then add the Turn servo to the mechanism.