GMAT Strategy

I've been studying for the GMAT since December, and I have generally tried to approach this as scientifically as possible. Here are my statistics. A "sample" is equal to 25 questions. And the mean is my average score over X number of those samples.

I have also taken 3 practice tests, 1 a month basically. In December I got a 570 before I started studying. In January I got a 720 and then in February I got a 710.

And today I took another practice test and instead of getting a score in the mid 700s like I expected after another month of heavy studying, I got approximately a 650. Quant raw: 23, Verbal raw 35. And I bombed across the board, not just on any particular section.

None of the test conditions have changed, I'm not feeling under the weather, etc.

Do I attribute this to a statistical anomaly? Or do I postpone the test (Scheduled for April)? Am I getting diminishing results from more studying? And if that's the case, what's an effective technique to address it?

Any advice/suggestions would be appreciated.

You could be forgetting information. Look at the practice test that you got the 650 on and compare it to the two tests where you scored above 700. Look to see if you are missing questions that you previously got right. If so, you're just forgetting info and you need to go back and review.

That's good advice, I did that and actually I came up with a different conclusion. On every test I mark questions that were hard for me with an 'H'. When I went back, I found that I got most of the hard questions on all of the tests correct. The mistakes that I made on this test and the previous ones were silly mistakes. Either I misunderstood the question (i.e. I gave the ratio for item 1 to total instead of item 1 to item 2) or I made a calculation error.

What I found was that I'm doing the questions too quickly, especially on this last test. I averaged 1:32 seconds per question for the Quantitative and 1:12 seconds on the Verbal.

So the problem it seems is that rather than forgetting information, I'm being careless and getting in my own way by rushing through the test.

Another thing I noticed t from reviewing them is that alot of those mistakes happened in the questions immediately following the ones marked 'H'. Basically, I got frustrated on the prior question and because of that I get careless on the following questions.

Does anybody have any test-taking or study tips to help address those two issues?