Are you certain that there’s no pattern to the questions you’re missing? The reason I ask is that in my experience, particularly in he Logical Reasoning section, students tend to rely exclusively on question type to discover (or not) patterns in their errors, and sometime legitimate patterns exist across question types. For instance (simplified example), a student whose grasp of causation is shaky may as a result miss a flaw question, an assumption questions and a weaken question on a section all for that same reason.
To more directly respond to your question, if you’re doing only timed practice, one thing you can start doing is a “blind review” in which you do the same sections both times and untimed. Here’s the way that works – after doing a section (or an exam) times, before checking your score, go back and do the same section (or test) untimed (this could be the next day, but doesn’t have to be all at once). As you’re scoring very well, you’ll probably keep the vast majority of your answers, but you’ll probably want to change some, too. Only after you’ve finished the untimed work should you check your answers, and thoroughly go over any that you missed (on either pass).
This has a couple of benefits – the untimed practice itself is useful, as is thinking about some of the questions in different ways. Additionally (and finally to your original point), the “recycling” will help the tests last longer.