I had the same problem with my OEM servo - only it did it before I tightened the steering = crap right out of the box and it got worse after I tightened the steering.
Some people use captured ball ends - which ones and where to find them appears to be another well kept secret - until I find some that fit and work I won't be writing about them.
My steering is frog jacked / the right side differs slightly from the left = not SYMETRICAL.
And the OEM chassis is warped.
The OEM servo is weak once the steering is tightened and slow as well.
I upgrade to a - Savox 1257 - BUT I had to make a new servo mounting block for one side because the HPI servo is way longer than anyone other servos.
Thanks Marv. Great info.
I only used the Hitec cause I had it and to eliminate the HPI servo as the problem which I thought I had. I did all the centering of the Servo horn as you described. the BEC Cap sounds like a good idea. Because both servos had problems centering with the servo arms disconnected I think I have probably answered my own question. I think the Servos coupled with the Spektrum RX may be too much for the BEC in the HPI ESC. Will try to confirm tonight.
I do need to finish off the front suspension as you have with the stabilization of the left servo post but just the RPM ends made a tremendous improvement when it did steer.
I noticed that you have your servo saver spring quite a ways up. I backed mine off till I could just see light between the bottom of the knob and the chassis to see if that helped servo centering and it didn't. Is there is an optimum location for it.