Actually, ROI is only one of my indicators and the bottom line is the monthly growth which it's hard to maintain. If I concentrate on ROI, I will limit my growth. Thus, I will not.
AdSense is the same as AM. If you're luckily to find one of the keywords which produces a high paid-out, expand on it to the max. Drop the low keyword paid-out. Look for the next one.
This has been answereed by sailorjwd but I want to add:
A paused or slowed down campain will normaly take place in a new account. A long established account doesn't have these problems on whole campaign but on the individual keyword or phrase only.