Hi ms04.
In my experience, when a person's verbal score is fluctuating as yours is, the reason is that the person is being too rule, pattern, and method driven, and not using logic enough.
What happens is that the rules, patterns, and methods that the person uses apply to some questions and don't work well for answering other questions. So, if you get a verbal section with a bunch of questions that your rules, patterns, and methods work for, you score high on verbal. If you get a verbal section in which most of the rules, patterns, and methods that you are using do not apply, you score low in verbal.
So, what the solution?
The solution is to adjust your approach to GMAT verbal. You have to realize that GMAT verbal is a tricky logic and reasoning game, one that you won't win by using gimmicky strategies or by looking for simple patterns. You have to seek to see more of what is going on in the questions and to use logical reasoning more. For instance, in answering a SC question, sure, knowing grammar rules helps, but the question is not testing your knowledge of grammar. It's testing your skill in seeing what is going on and in noticing which version of the sentence is most logically constructed. So, your focus has to be on learning to see what's going on.
In order to do so, when you do verbal practice questions, do them slowly, seeking to see what you have to see in order to get them right, and shooting for a very high hit rate.
The rules and the explanations are not really the point. You can learn dozens of rules and read 1000 verbal question explanations and still not score high on the verbal section, because reading explanations does not really help you to develop the key skill that you need in order to score high in verbal, skill in seeing what is going on. So, as you are practicing, for every question, as yourself, "What do I have to see in order to get this one correct?" and if you don't get one, ask yourself, "What would I have had to see in order to get this one correct?"
Also, if you have not already, try using official GMAT Prep practice tests from mba.com. The verbal questions on those tests are generally particularly well constructed. So, you may score higher on verbal on those tests than you have been on non-official tests.