Hi daletang,
First off, a 650 is still a solid score (it's right around the 80th percentile overall), so it might be enough to help you get into your first-choice Business School. Given your practice CAT scores, it's understandable why you would want to retest though. Before you rush back in to retake the GMAT though, I'd like to know a bit more about how you took your CATs:
1) Did you take the ENTIRE CAT each time (including the Essay and IR sections)?
2) Did you take them at home?
3) Did you take them at the same time of day as your Official GMAT?
4) Did you ever do ANYTHING during your CATs that you couldn't do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
5) Did you ever take a CAT more than once?
It's possible that you just had a "bad day"; but if we can figure all of this out quickly, then you shouldn't wait too long to retest.
GMAT assassins aren't born, they're made,
Rich