Sometimes I get knee pains. What causes this and how do I prevent it?!

So, about once or twice a month I get dull aches in one or both of my knees. It's usually in the evening, say 8:00 pm or so, and usually a good sleep will take care of them for another fortnight or so before they act up again.
I'm a 6'2" male at about 200 lbs, so it could be a weight issue, but if it were, don't you think it would be more of a regular thing? What do you think could be causing this?
Thanks for any help you can provide. ;)