I'm constantly tweaking my campaigns because something changes every day. How's the quality score look on the keywords that are starting to slip? If they're not 'Great' then keep improving your ads to get a higher CTR, then your bid prices will fall.
If they're already at a high QS, then start weeding out non-converting keywords that are sucking money.
That's the thing.. QS is not an issue at all.
The problem is that they've been performing great for a few months and now are starting to slip quite a bit. So I'm wondering if I should ride it out and see if it gets better or tank bids to stay in positive ROI.