the only thing is, bouncing SERPs sounds more like an Algorithm issue than a penalty issue to me
I think it's an automatic filter. There seems to be loads of them just lurking in the SERPs, and you trip them when you reach a certain position or level of click-throughs in SERPs.
OP says he lost rankings a day after he achieved site-links in his listing. Site links are usually achieved once you become an authority (judged by incoming external links). Once he got that status a filter was automatically run, and his site failed one of the tests. Most of these filters are based on on-page issues, but I think there are a few triggered by off page stuff.