I started getting that the summer of 2011. Sometimes the patch of bubbles would be about 6" x 3"! It would happen more in the morning and like you said, SOMETIME throughout the day. I never knew how long they lasted because I'd flush. I had my urine tested, etc (Just had routine blood work a month before that) and everything was normal. I recall getting scared because I was thinking, "Oh maybe it's just something in the toilet..." but then when I went in the cup at the doctors there was a small patch of bubbles in it! Turns out it was nothing at all. The doctor said sometimes the stream becomes turbulent (but it would happen even with a slower stream), she just said it's the way it is hitting the water when coming out. And you are a guy, so you would be hitting the water from a lot greater distance. When I was telling my friend she said her son always gets bubbles and so does my husband. I felt it was normal for a GUY so that's why I was worried. But it was nothing.
Once I had more of a "foamy" patch, but that was because I didn't drink water ALL day and I was a bit dehydrated.