I used both Kaplan and AAMC tests during my studying. I took 5 of each. The Kaplan tests seem to have a more generous grade distribution than AAMC. For example, say you got 8 questions wrong and got an 11 on Kaplan, 8 questions wrong might get you a 9 or 10 on AAMC (note: those are made up numbers for illustrative purposes only). My AAMC scores were consistently about 2 points lower than my Kaplan scores. Before taking the MCAT, I averaged the 10 practice tests and scored the same on the real MCAT as the average of my 10 practice tests. In other words, I did a little bit better than my AAMC scores on the real test and a little worse than my Kaplan scores. I think doing a mix of the tests is the best idea. Opinions will likely vary as everyone is different in their prep though.
EDIT: as far as diagnosing what is going on with your tests, after each test you should look at all the things you got wrong and see if you can find a pattern. The post mortem on each practice test is essential, otherwise you can't expect to get better.