This is why pasta instructions will tell you to add the pasta only when the water is at a vigorous boil. Either you stir, or you have the pasta boiling like it's a volcano about to pop.

There was a neat cooking blog where an italian fella went into all the 'old myths' that his grandmother used to tell him: Vigorous boiling; Large Pot; when to add sauce; etc. It all boiled down to knowing what you're cooking. Pasta is starchy, so if it sits in hot water instead of being tumbled about, all the starch that's been liberated into the water settles back on the pasta (and not in a good way).