Usually, such a "bounce" is caused by dirty connections. If you don't have a good connection, then the wiring will not deliver full voltage to the servo. Unplugging and reconnecting the servo several times usually cleans the connectors.
When you mention that the servo sometimes has no holding power, it means that it's getting no power. We'd suspect the wiring has a problem somewhere.
Hello Bax, thanks for your answer.
I'll check the connectors.
Yesterday I noticed that after a couple of flights the problem disappear.
Anyway I'll check the ailerons servo connectors inside the wing.