Sounding to me like the position sensing pot has gone bad... or is flopping inside the servo case.
Definitely an unsafe servo to use whatever is wrong wth it. Replace before it puts your airplane in the dirt. (or worse... it crashes the plane ito you or someone else.)
Get a Tower TS-53. Peel the label and you raise its value $5 (its a Futaba 3003 with a label stuck on. TX-53 = $9... S-3003 = $14 Tower pays you to let them label the servo...

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