Whenever I've tried to cook oatmeal in the microwave (and I always use the old-fashioned stuff), I do it at 50% for 5 minutes. That's what my directions say to do, and I've never seen the 2.5 - 3 min that yours say. But, every single time, they boil over the top of the bowl - oatmeal just makes lots of bubbles, and it overflows, even though the power goes down every so often on 50%.

So I eventually gave up and just cook it on the stove now. It takes the same amount of time on the stove, and you don't have to clean the microwave. I've occasionally had luck with using a large bowl in the microwave (like one of my glass mixing bowls, either the large or the medium), but that's still spotty. And then I've got a huge bowl to wash out. :P