Yes, I've seen this kind of thing. The reason it periodically twitches is because I put a bit of code in Marlin to "refresh" the servo position periodically. I found that during printing, sometimes it would "droop" and so I wanted something to keep pushing it back to where it is supposed to be every once in a while. So that's that part.
But, your servo should be going to 90 degrees at startup, not 45, so I think you have an offset issue. Then when you extend it, it can't make it to the (what it considers) 0 degree position, so there's some bounce-back when the servo is disabled. You can adjust that by changing the angles in Marlin (unfortunately you can't do that from the LCD you have to recompile the source). But it's just one line of code, in the file configuration.h,
#define SERVO_ENDSTOP_ANGLES ...
The third set of numbers will say 0,90. Try 10,100 or something and you should see the servo settle a bit.
And lastly, something I just recently discovered is that the servo power supply can be tweaked up a bit to make it more responsive (and possibly increase the torque). Hook your dmm up to the output of that little DC:DC that powers the servo, and turn the tiny pot on the board cockwise about a half turn and try setting it to 5.5V instead of 5V. Just don't exceed 6V or it will kill the servo.
Hope that helps!