I've seen other threads with this problem, might be worthwhile to search around. I can't find it again, but I've seen them before.
Tech's suggestions are good though, it really all depends on what you have open.
As for the previous threads that I've seen, I don't recall them having a definitive solution, sorry.
Again, look at tech's points, and search around for more answers. I'm sure there's something amiss that's causing this problem...