Hi KillerGMAT,
The 750+ score is approximately the 98th percentile - meaning that 98% of Test Takers either CAN'T or WON'T do what it takes to score at that level. Thankfully, NO Business School actually requires a Score that high. This is meant to say that the score that you "want" and the score that you "need" are likely not the same thing.
Test Day is a rather specific 'event' - the details are specific and they matter, so you have to train as best as you can for all of them. The more realistic you can make your CATs, the more likely the score results are to be accurate. The more you deviate, the more "inflated" your practice scores can become - and that's likely what happened here. By skipping sections, taking the CATs untimed, retaking CATs that you've already taken, etc., you weren't properly training for the FULL GMAT 'experience' - by extension, these CAT scores are likely not accurate.
Test Day involves a variety of really specific steps and parameters (including steps before the Test even begins - such as leaving your home, traveling, etc.). Every factor matters, including the psychological ones. When you sit down on Test Day, you KNOW that you're going to be in the Computer Lab for about 4 hours - but if you're just taking individual sections (or taking a CAT without the Essay and IR sections), then you KNOW that you'll be done in 1-2 hours. The attitude and energy that you use during practice will NOT be a match for what you'll need on Test Day, so it's not a proper way to practice.
Thankfully, these are relatively easy issues to fix - you just have to take the steps to take your CATS in a more realistic fashion (take the FULL CAT - with the Essay and IR sections, take it away from your home, at the same time of day as when you'll take the Official GMAT, etc.). To determine the exact areas you should be working on next, we need to properly assess your current skills, so I suggest that you plan to take a NEW FULL-LENGTH CAT sometime soon (perhaps this weekend). Once you have that score, you should report back here and we can discuss how best to proceed.
GMAT assassins aren't born, they're made,
Rich