This could be an electrical problem but you will need someone with electrical expertise to answer that particular avenue for you if that is the issue (i.e the coil is not getting enough voltage making it weak, rather than the coil itself being weak). Parts even on new games do go bad though, it may just be the case that the coil is bad and it needs replacing. If you can get a new coil under warranty for nothing might be worth getting one anyway.

Historically this has been an issue with the alignment of the auto plunger arm. It happened to ACDC a lot. I've never had to fix it myself, but you just have to get under there and check that the arm is lined up properly. There was also a fix a few people did adding a small spacer or washer in there to help stabilise it. Try searching for the issue under ACDC auto plunger.

https://pinside.com/pinball/forum/topic/acdc-auto-plunger-is-starting-to-get-weak

There you go.

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Mine too has started to get a bit weak, though not as bad as yours. Mine makes it around the orbit or up to the pops (depending on if the gate is open), just a bit slowly. Similarly, the one time I opened it and played with it, it started working well again for a while. It is now slowing down again a bit, but just a bit. I wonder if it could be the coil sleeve. My alignment looks ok, but perhaps it needs a slight tweak. Interested to hear others experiences with this.

Maybe the wire strands are breaking inside?

btw, this would not be detected by voltage testing.

Try tugging on them near the coil lugs, and see if you have thinning.

Email Stern about it. They were quick to answers my emails and resolve a different issue with my new ST.