@Jen the ones that are a month old are probably at the top because they've been "revived" because someone just added something to the thread. If you want just threads ordered by when they were originally started try "sort by START"

Yeah the jumping around thing is unsettling huh ... that's all the new stuff flowing in if you're at the top of the page and there's been updates. It's not broken, it's just doing it's darndest to get things up to date and in order. Poor design and needs some love!