It's possible that you have some genuine GMAT issues to work on, but I'm not sure that your practice test scores mean what they seem to mean.
For starters, it's always wise to take all practice test scores with a serious grain of salt, unless you're doing a GMATPrep test. It's particularly impossible for test-prep companies to write realistic verbal questions, so the verbal scores are especially suspect.
So when I see a dropoff from your Knewton scores to your MGMAT scores, I don't think it really tells us anything. Your quant scores are reasonably consistent across the eight tests, and it's really just the verbal scores that are bouncing around. And that's exactly what I would expect if you take eight non-official practice tests from three different companies.
If you're really worried about losing your verbal sharpness, do a big fat wad of official verbal questions (from your choice of the OG, verbal guide, and/or GMATPrep Question Pack) until you get your mojo back. If you're worried about stamina, take on some LSAT CR
and RC sections--they'll really test your ability to focus. And if you do well on all of that stuff, go ahead and take a stab at a GMATPrep test. It looks like you might have some legit quant weaknesses that you'll want to keep working on, but chances are good that the verbal issues are largely a result of overusing non-official practice tests. Stick to the official stuff, and you'll probably be fine.
Helping students kick the GMAT in the nuts since 2002... www.gmatninja.com.