I don't think I'm altering the question.
That's the metric to use. As long as you aren't altering the intent of the question then it's fine to make an edit that improves the post and its readability. That's not to say that more substantial edits that introduce questions or guess at intent are always wrong, I only mean that the type of edit you describe is always fine. I call these minor edits but that term is very loosely defined and some consider only spelling and grammar edits to be minor.
So in short, yes, it's encouraged to make these kinds of edits. Highlighting the core question(s), clarifying the post and improving readability are all within the scope of an edit.
If the OP described problematic behaviour and simply left out the question ("How do I get X to stop happening"), it's fine to add that in, since it's reasonable to assume that that's his goal. Sometimes people get hung up on the lack of a question in a post and Vote To Close when they could just add it in.
If you can't clearly determine what the question is, you probably shouldn't guess at it because the OP might revert it, invaliding existing answers. Jane's answer mentions this but I wanted to point out that this type of major edit should only rarely be done. Most of the time you should leave a comment asking the OP to clarify and vote to close it as unclear to prevent answers from users who interpreted the question incorrectly.
I do sometimes make such a major edit when I'm reasonably confident that I got what the OP is asking, but for most such questions a comment and close vote is more appropriate.