OK...just as I thought. Your power supply is not big enough. The other thing is you may need to get a larger servo to hold the turret vertically. The oscillation you see is the servo struggling to hold position and causing voltage dips because it is stalling. Get a 3A power supply from surplus, use several voltage regulators in parallel, filter with caps and get a metal gear servo to hold up the turret, they tend to be more heavy duty.
Nick
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