A friend bought 8 SG-90's off E-Bay new about 2 months ago, had nothing but problems. All 4 servos he installed in one plane had a bad problem of servo drift. One servo was noisy and jittery. I agreed to help and observed the same on both my 72 MHz system Futaba generic that has been rock solid and my newer Futaba 6EX 4.2 GHz radio. I experienced the exact same with those servos. One would center the control arm. Shut the radio off. Turn it back on, the neutral drifted several degrees. Then I rapidly exercised the servos by rapidly moving the sticks back and forth rapidly 20 times. The neutral drifted further. After 10 exercises each function, three of the servos had drifted about 22 degrees.
I'm thinking that the spindle shaft from the feedback potentiometer is not seated firmly enough to the gear its connected to, causing the shaft to slip a little each time, but I haven't disassembled the servos to be sure.
He told me that you get what you pay for. The metal geared version worked fine for him, but not this recent lot of budget servos. He replaced them with more expensive branded servos.