Hi Sylvester.
While non-official practice tests can provide good practice, there can be some difference between official practice tests and non-official practice tests in terms of both the form of the questions on the tests and the ways in which the tests are scored. So, the fact that your scores are fluctuating can likely be explained in part by the fact that you have been switching back and forth between test providers.
Notice also, that your recent quant score on an official practice test, Q44, is not that far from your previous quant score on an official practice test, Q46. So, if, to be consistent, we compare official with official, we see that your quant score has not actually fluctuated that widely.
Regarding what's going on, it's tough to say much just looking at test scores. What you said about your test experiences helps fill in the picture.
Going by your scores and what you said, regarding quant, it appears that you both have some areas of quant in which you could be stronger and have had some time management issues. Of course strengthening less strong areas helps to address time management issues. You may have addressed the issues you faced on the quant section of your most recent official test by working on accuracy and time management, but we'll know better once you take another official practice test. If you don't hit your quant score target on that one, you may need to work go beyond working on time management and accuracy to working more on some specific quant topics in which you could be stronger.
Regarding verbal, the V42 may not be a fluke, and you may continue to score at that level. If, on the other hand, your verbal scores continue to fluctuate, then it's likely that you have to tighten up your approaches to answering verbal questions so that you more reliably get verbal questions correct. Once again, it's hard to say what the case is since you have been achieving different scores on tests from different providers. So, I'm curious to see how you do on your next official practice test.
For some insights into how to tighten up your approaches to answering verbal questions, see this post.
How to Score High on GMAT VerbalRegarding whether you should purchase tests 5 and 6 if you hit your target on 3 and 4, maybe not. You've taken a bunch of practice tests already. So, it's not as if you don't have experience in handling the test itself, and you could save 5 and 6 in case you don't hit your score goal when you take the actual GMAT.