I had a similar problem at the beginning. For my first two GMAT attempts, I was getting 710, 720 from my GMATPrep mock exams. On the actual days, I came away with only 640, 660. The things affecting my performance were:
1) Anxiety (you may not feel very anxious but it could still be hurting your performance); and
2) Timing.
Anxiety:At first, I tried to rush through the GMAT-taking process, trying to make certain MBA application deadlines. This was a mistake, as I wasted two GMAT attempts doing this. The fact that you're trying to get over the GMAT process quickly and move on, is probably giving you a certain level of stress, though you may be unaware of it. My approach, though it may not be the right one for you, was to completely put aside all GMAT and MBA matters for 6-8 months and get back to my normal life. I wanted to reduce the importance that the GMAT had to me, then return with a chilled attitude and clear mind.
I also noticed that because I tried to prepare special healthy snacks, avoid caffeine, do things that I wouldn't normally do (or avoid doing things that I normally did), to try and be on my best form for D-Day, I inadvertently created more stress for myself. This was because everything I did was geared towards the exam day and it tricked my mind into a do-or-die mode. So, for my third and final attempt, I didn't do anything special. No vitamins, no energy bar, nothing I wouldn't normally do or not do. I basically didn't sweat the small stuff. I also told myself that I was more than adequately prepared, I'd most probably do well but otherwise, could always do the GMAT again - it wasn't a big deal. And I also stopped being such a perfectionist (in line with not sweating the small stuff, above) - I told myself not to bother too much about the AWA or IR sections and save my brain for the meaty part of the exam.
Timing:This is the other big killer. To fix my problem for good, I practised GMATPrep mock exams. Again. And again. And again. I purchased the additional exam pack too, so I had something like 6 exam papers to do, on a loop. Don't bother with Kaplan,
MGMAT or any other mocks (I found Kaplan 800's questions a doss and I can assure you that I'm not Missy Brain, so I threw the book out), because they're not even remotely the same as the actual test that you'd be taking. Stick to GMATPrep. The better you get, the more difficult questions the same few GMATPrep exams are going to throw at you. Do these mocks daily, to get the hang of timing and to get your mind used to the whole process. Don't forget to keep doing what you're already doing, in terms of general revision. I wrote out over a hundred flashcards with questions from the Official GMAT Guide that I got wrong and tested myself whenever I had a spare moment. Also, because I kept forgetting mathematical formulas and rules, I wrote out a 3-page "cheat sheet" and went over it daily.
Leading up to my third attempt, I was getting 750, 760 consistently from my GMATPrep practice exams. On the actual day, since I don't snack or drinks sugary stuff, I went in with just a banana and a bottle of water. I came away with 740, which was still 10-20 points lower than what I'd been getting in practice exams but I knew my real score would always drop a little, from actual-day stress. Ultimately, I think the above approach helped to close the gap between practice and test day results, and I hope it helps you too.
***Edit: To avoid any trace of ambiguity, I meant that, in my experience, no other CATs save GMATPrep's comes close to simulating the GMAT. Otherwise,
Manhattan GMAT does run a very solid course for both verbal and quant and this was what helped me to establish my basics. I chose
MGMAT for this purpose (establish my foundations), because of others' reviews online and can wholeheartedly recommend them for a comprehensive approach to tackling GMAT topics.