Ooooh yum, that PB idea sounds good. I just can't over overboard with it!
And about the cornstarch...did you let the oatmeal sit for a couple minutes after you cooked it to let it "set"? I guess everyone makes theirs different, but I always added hot water (instead of cooking it over the stove) to my oatmeal, put a paper towel over it, and let it sit a couple minutes. Maybe try that?? Hope that helps.