That's pretty much the pattern I've been seeing with several of my clients recently. The instability is getting worse so either Google are slacking on the filters and forever twiddling with them or they are deliberately making it harder to SEO against them. Even my recent bounce effect article is starting to look dated.

As for PR, it's a very vague indicator that doesn't directly translate to rank which is why I gave up chasing PR a long time ago. It's still good to see the odd bit of green though, just to keep being sure Google knows you exist.