It seems this is a common problem with the Stern autolauncher. I don't know if X-men uses the same mech but here are some threads for weak ACDC autolaunchers.
http://pinside.com/pinball/forum/topic/acdc-pro-ball-plunger-getting-weak
http://pinside.com/pinball/forum/topic/acdc-auto-plunger-is-starting-to-get-weak