GMAT Strategy

I have the worst study habits ever, so I figured I would simply take a lot of practice tests to prepare for the GMAT. So I started taking them a few weeks ago and scheduled my test date for May 31st. I've been taking practice tests, but have been unhappy with my score (am aiming for at least 710). I was thinking about postponing my test date based on my low scores, but I took the mba.com test today and did much better than I expected. I'm trying to decide whether it was a fluke and if I should still delay my test date ... I'm nervous because I don't understand how there can be such huge point swings! Here is my practice test history so far, in the order that I took the tests:

Princeton Review CAT - 690
800score.com Test #1 - 700
Kaplan Paper - 720
800score.com #2 - 640
Manhattan GMAT - 640
800score.com #3 - 660
mba.com #1 - 770

So I'm looking at a massive, 130-point swing! In particular, I took those last three tests on consecutive days (Saturday, Sunday, and tonight).

I'm going to do a lot of studying between now and the test either way, but should I postpone my test date? These massive swings are scaring me and making me think that the 770 was a total fluke.

Any and all advice would be appreciated!