PLEASE HELP! NEED ADVICE!

Hello guys,

I took the GMAT 3 months ago and scored 590 (Q46, V27). During my preparation I studied with the OG, Kaplan, took all Kaplan´s CATS and the 2 free GmatPrep Tests.

About a month ago I realized I had to do something about my score if I wanted to get into a good school, so I bought the Online Practice CAT Exams & Question Banks package from Manhattan and this are my results:

CAT 1: 490 (4 months ago, this is the free one I took before taking my only official GMAT)
CAT 2: 360 (Q28, V4) – 3 weeks ago, i quit during Quant because I realized I needed more practice.
CAT 3: 530 (Q33, V30) – 2 week ago
CAT 4: 360 (Q28, V4) – 2 week ago (had problems with my internet and couldn’t finish)
CAT 5: 620 (Q42, V33) – last week
CAT 6: 580 (Q45, V25) – a couple of days ago, I am almost sure I got much harder questions than on CAT 5, is this normal?

My best score is 620, and my target is 650 so I am going to keep studying a week or so and take another CAT.

My questions:

1. As you can see, I practically lost 2 MGMAT CATS (2 and 4). Should I reset the tests and try it again? Will the score be inflated?

2. I took the 2 free GmatPrep tests like 4 months ago, but I noticed they released a new version. Should I take these tests? Will the results be accurate?

3. I want to apply to the University of Texas at Austin, Mccombs. I was told that if I get a 630 I have pretty good chances on being accepted. What do you guys think?

4. Should I buy other tests? Which ones do you recommend?

5. Do you really think that with a 630 I could get into a Mccombs or maybe Rice?

Background:
Engineer, 25 years old, from Venezuela. Since I graduated I have been working in INELECTRA, the biggest engineering and construction company in Venezuela, focused in oil and gas. Right now I have been in INELECTRA for 2 years and by the time I expect to start my MBA (by the end of next year) I will have about 3 years experience.

I would really appreciate your advice.