heyyy,
Yes, I also saw fluctuations when I was doing practice exams, haha. The first time I did a practice exam, I happened to get 410 in QR, and now my score is 710 in my most recent practice test (which is an improvement, but I would ideally like to score higher...). The main thing I changed was just making sure I stuck to the time limit, i.e. VR has 21 mins for 44 questions!!, so I made sure to practice my speed reading + allocating a max of 40-45 secs per question (yes it's pretty hard given that you have 4 questions that relate to one text)
In between the subsections - when you have the one-minute break, I suggest you recuperate yourself and quickly calculate how long you should spend on each question. This ensures that you don't spend, for example, 5 mins reading a text or even thinking about whether or not to click numbers using the dodgy calculator (for questions in DM and QR that required the calculator, I would try to use approximate numbers instead of opening it up - which does waste HEAPS of your time).
pro tip: if you get stuck on a question and are going WAYY over the allocated time, click a random answer, flag, and move on with no regrets - you are guaranteed a 25% chance of getting it right!!
Lastly, just a quick q did you happen to do the UCAT M.E.D. Mock exam? I personally found it really helpful in identifying my weak spots (for me, it was VR and SJT)